Derive separate keys for each direction.

api/net/sf/briar/api/crypto/CryptoComponent.java

components/net/sf/briar/crypto/CryptoComponentImpl.java

...ents/net/sf/briar/transport/ConnectionRecogniserImpl.java

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

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

test/build.xml

test/net/sf/briar/FileReadWriteTest.java

test/net/sf/briar/crypto/CryptoComponentTest.java

.../net/sf/briar/transport/ConnectionRecogniserImplTest.java

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