Added a flag to indicate the last frame of the connection.

api/net/sf/briar/api/protocol/ProtocolWriter.java

api/net/sf/briar/api/transport/TransportConstants.java

components/net/sf/briar/protocol/ProtocolWriterImpl.java

...onents/net/sf/briar/protocol/duplex/DuplexConnection.java

.../sf/briar/protocol/simplex/OutgoingSimplexConnection.java

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

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

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

...t/sf/briar/transport/IncomingAuthenticationLayerImpl.java

...riar/protocol/simplex/SimplexConnectionReadWriteTest.java

test/net/sf/briar/transport/ConnectionReaderImplTest.java

test/net/sf/briar/transport/ConnectionWriterImplTest.java

...t/sf/briar/transport/IncomingEncryptionLayerImplTest.java