More DB support for private messages - needs unit tests.

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

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

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

...nents/net/sf/briar/db/ReadWriteLockDatabaseComponent.java

...onents/net/sf/briar/db/SynchronizedDatabaseComponent.java

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

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