Factored out the database cleaner.

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

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

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

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

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

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

test/build.xml

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

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