Moved MessageHeader into DB component and added read/starred flags.

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

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

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

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

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

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

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

components/net/sf/briar/db/H2Database.java

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

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

...nents/net/sf/briar/protocol/MessageHeaderFactoryImpl.java

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

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

test/net/sf/briar/db/H2DatabaseTest.java