Application layer keepalives to detect dead TCP connections.

DuplexOutgoingSession flushes its output stream if it's idle for a
transport-defined interval, causing an empty frame to be sent. The TCP
and Tor plugins use a socket timeout equal to twice the idle interval to
detect dead connections.

See bugs #27, #46 and #60.

briar-desktop/src/org/briarproject/plugins/bluetooth/BluetoothPlugin.java

@@ -78,6 +78,11 @@ class BluetoothPlugin implements DuplexPlugin {
 		return maxLatency;
 	}
 
+	public long getMaxIdleTime() {
+		// Bluetooth detects dead connections so we don't need keepalives
+		return Long.MAX_VALUE;
+	}
+
 	public boolean start() throws IOException {
 		// Initialise the Bluetooth stack
 		try {