Try to remove some locking

Ideally we would be able to avoid locking at all on the main thread, but
needs must.

src/main/java/dan200/computercraft/shared/peripheral/modem/ModemPeripheral.java

@@ -26,7 +26,7 @@ import static dan200.computercraft.core.apis.ArgumentHelper.getInt;
 public abstract class ModemPeripheral implements IPeripheral, IPacketSender, IPacketReceiver
 {
     private IPacketNetwork m_network;
-    private Set<IComputerAccess> m_computers = new HashSet<>( 1 );
+    private final Set<IComputerAccess> m_computers = new HashSet<>( 1 );
     private final ModemState m_state;
 
     protected ModemPeripheral( ModemState state )
@@ -58,7 +58,7 @@ public abstract class ModemPeripheral implements IPeripheral, IPacketSender, IPa
         setNetwork( getNetwork() );
     }
 
-    public synchronized void destroy()
+    public void destroy()
     {
         setNetwork( null );
     }
@@ -68,7 +68,7 @@ public abstract class ModemPeripheral implements IPeripheral, IPacketSender, IPa
     {
         if( packet.getSender() == this || !m_state.isOpen( packet.getChannel() ) ) return;
 
-        synchronized( this )
+        synchronized( m_computers )
         {
             for( IComputerAccess computer : m_computers )
             {
@@ -84,7 +84,7 @@ public abstract class ModemPeripheral implements IPeripheral, IPacketSender, IPa
     {
         if( packet.getSender() == this || !m_state.isOpen( packet.getChannel() ) ) return;
 
-        synchronized( this )
+        synchronized( m_computers )
         {
             for( IComputerAccess computer : m_computers )
             {
@@ -201,21 +201,33 @@ public abstract class ModemPeripheral implements IPeripheral, IPacketSender, IPa
 
     @Override
     public synchronized void attach( @Nonnull IComputerAccess computer )
+    {
+        synchronized( m_computers )
         {
             m_computers.add( computer );
+        }
+
         setNetwork( getNetwork() );
     }
 
     @Override
     public synchronized void detach( @Nonnull IComputerAccess computer )
+    {
+        boolean empty;
+        synchronized( m_computers )
         {
             m_computers.remove( computer );
-        if( m_computers.isEmpty() ) setNetwork( null );
+            empty = m_computers.isEmpty();
+        }
+
+        if( empty ) setNetwork( null );
     }
 
     @Nonnull
     @Override
-    public synchronized String getSenderID()
+    public String getSenderID()
+    {
+        synchronized( m_computers )
         {
             if( m_computers.size() != 1 )
             {
@@ -228,3 +240,4 @@ public abstract class ModemPeripheral implements IPeripheral, IPacketSender, IPa
             }
         }
     }
+}

src/main/java/dan200/computercraft/shared/peripheral/modem/WirelessModemPeripheral.java

@@ -21,6 +21,12 @@ public abstract class WirelessModemPeripheral extends ModemPeripheral
         m_advanced = advanced;
     }
 
+    @Deprecated
+    public WirelessModemPeripheral( boolean advanced )
+    {
+        this( new ModemState(), advanced );
+    }
+
     @Override
     public boolean isInterdimensional()
     {

src/main/java/dan200/computercraft/shared/peripheral/modem/WirelessNetwork.java

@@ -13,8 +13,9 @@ import dan200.computercraft.api.network.IPacketSender;
 import dan200.computercraft.api.network.Packet;
 
 import javax.annotation.Nonnull;
-import java.util.HashSet;
+import java.util.Collections;
 import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 
 public class WirelessNetwork implements IPacketNetwork
 {
@@ -34,45 +35,34 @@ public class WirelessNetwork implements IPacketNetwork
         s_universalNetwork = null;
     }
 
-    private final Set<IPacketReceiver> m_receivers;
+    private final Set<IPacketReceiver> m_receivers = Collections.newSetFromMap( new ConcurrentHashMap<>() );
-
-    private WirelessNetwork()
-    {
-        m_receivers = new HashSet<>();
-    }
 
     @Override
-    public synchronized void addReceiver( @Nonnull IPacketReceiver receiver )
+    public void addReceiver( @Nonnull IPacketReceiver receiver )
     {
         Preconditions.checkNotNull( receiver, "device cannot be null" );
         m_receivers.add( receiver );
     }
 
     @Override
-    public synchronized void removeReceiver( @Nonnull IPacketReceiver receiver )
+    public void removeReceiver( @Nonnull IPacketReceiver receiver )
     {
         Preconditions.checkNotNull( receiver, "device cannot be null" );
         m_receivers.remove( receiver );
     }
 
     @Override
-    public synchronized void transmitSameDimension( @Nonnull Packet packet, double range )
+    public void transmitSameDimension( @Nonnull Packet packet, double range )
     {
         Preconditions.checkNotNull( packet, "packet cannot be null" );
-        for( IPacketReceiver device : m_receivers )
+        for( IPacketReceiver device : m_receivers ) tryTransmit( device, packet, range, false );
-        {
-            tryTransmit( device, packet, range, false );
-        }
     }
 
     @Override
-    public synchronized void transmitInterdimensional( @Nonnull Packet packet )
+    public void transmitInterdimensional( @Nonnull Packet packet )
     {
         Preconditions.checkNotNull( packet, "packet cannot be null" );
-        for (IPacketReceiver device : m_receivers)
+        for( IPacketReceiver device : m_receivers ) tryTransmit( device, packet, 0, true );
-        {
-            tryTransmit( device, packet, 0, true );
-        }
     }
 
     private void tryTransmit( IPacketReceiver receiver, Packet packet, double range, boolean interdimensional )