Separated MessageHeader interface from Message interface.

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

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

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

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

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

test/net/sf/briar/ProtocolIntegrationTest.java

test/net/sf/briar/db/TestMessage.java

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

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