Simplify Database methods, move logic to DatabaseComponent.

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

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

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

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

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

components/net/sf/briar/plugins/PluginManagerImpl.java

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

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

test/net/sf/briar/plugins/PluginManagerImplTest.java