Removed the restriction that transport updates have to be written in

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

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

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

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

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

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

...ts/net/sf/briar/protocol/writers/TransportWriterImpl.java

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

test/net/sf/briar/serial/ReaderImplTest.java