Pass segments rather than frames to/from segmented plugins.

api/net/sf/briar/api/plugins/Segment.java

api/net/sf/briar/api/plugins/SegmentSink.java

api/net/sf/briar/api/plugins/SegmentSource.java

...pi/plugins/duplex/DuplexSegmentedTransportConnection.java

.../api/plugins/simplex/SimplexSegmentedTransportReader.java

.../api/plugins/simplex/SimplexSegmentedTransportWriter.java

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

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

...nents/net/sf/briar/transport/ConnectionEncrypterImpl.java

...s/net/sf/briar/transport/ConnectionReaderFactoryImpl.java

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

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

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

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

.../net/sf/briar/transport/SegmentedConnectionDecrypter.java

.../net/sf/briar/transport/SegmentedConnectionEncrypter.java

test/build.xml

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

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

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