Minor refactoring.

api/net/sf/briar/api/db/DatabaseComponent.java

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

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

api/net/sf/briar/api/protocol/writers/AuthorWriter.java

api/net/sf/briar/api/protocol/writers/GroupWriter.java

.../sf/briar/api/protocol/writers/ProtocolWriterFactory.java

.../briar/api/protocol/writers/SubscriptionUpdateWriter.java

.../sf/briar/api/protocol/writers/TransportUpdateWriter.java

components/net/sf/briar/db/DatabaseComponentImpl.java

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

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

...nents/net/sf/briar/protocol/writers/AuthorWriterImpl.java

...onents/net/sf/briar/protocol/writers/GroupWriterImpl.java

.../sf/briar/protocol/writers/ProtocolWriterFactoryImpl.java

.../net/sf/briar/protocol/writers/ProtocolWritersModule.java

.../briar/protocol/writers/SubscriptionUpdateWriterImpl.java

.../sf/briar/protocol/writers/TransportUpdateWriterImpl.java

...net/sf/briar/transport/batch/OutgoingBatchConnection.java

...nents/net/sf/briar/transport/stream/StreamConnection.java

test/net/sf/briar/ProtocolIntegrationTest.java