BATCH_ID and MESSAGE_ID don't need to be structs.

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

api/net/sf/briar/api/serial/SerialComponent.java

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

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

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

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

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

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

components/net/sf/briar/serial/SerialComponentImpl.java

test/net/sf/briar/protocol/AckReaderTest.java

test/net/sf/briar/protocol/ProtocolWriterImplTest.java