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/modem/ModemPlugin.java

@@ -61,6 +61,11 @@ class ModemPlugin implements DuplexPlugin, Modem.Callback {
 		return maxLatency;
 	}
 
+	public long getMaxIdleTime() {
+		// FIXME: Do we need keepalives for this transport?
+		return Long.MAX_VALUE;
+	}
+
 	public boolean start() {
 		for(String portName : serialPortList.getPortNames()) {
 			if(LOG.isLoggable(INFO))