Expose serialisation overhead without breaking encapsulation.

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

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

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

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

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

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

components/net/sf/briar/protocol/writers/AckWriterImpl.java

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

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

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

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

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

test/net/sf/briar/protocol/writers/ConstantsTest.java