Exposed message handling methods through DatabaseComponent interface.

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

...r-api/src/net/sf/briar/api/db/NoSuchMessageException.java

briar-core/src/net/sf/briar/db/Database.java

briar-core/src/net/sf/briar/db/DatabaseComponentImpl.java

briar-core/src/net/sf/briar/db/JdbcDatabase.java

briar-tests/src/net/sf/briar/db/DatabaseComponentTest.java

briar-tests/src/net/sf/briar/db/H2DatabaseTest.java