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-api/src/org/briarproject/api/plugins/duplex/DuplexPlugin.java

@@ -7,6 +7,9 @@ import org.briarproject.api.plugins.Plugin;
 /** An interface for transport plugins that support duplex communication. */
 public interface DuplexPlugin extends Plugin {
 
+	/** Returns the transport's maximum idle time in milliseconds. */
+	long getMaxIdleTime();
+
 	/**
 	 * Attempts to create and return a connection to the given contact using
 	 * the current transport and configuration properties. Returns null if a