Expose the encryption and authentication overhead without breaking

encapsulation.

This should allow callers to calculate maximum packet sizes without
knowing the details of the transport protocol.

components/net/sf/briar/transport/ConnectionEncrypter.java

@@ -11,4 +11,11 @@ interface ConnectionEncrypter {
 
 	/** Encrypts and writes the MAC for the current frame. */
 	void writeMac(byte[] mac) throws IOException;
+
+	/**
+	 * Returns the number of encrypted bytes that can be written without
+	 * writing more than the given number of bytes, including encryption
+	 * overhead.
+	 */
+	long getCapacity(long capacity);
 }