FileStore.compact does not persist compacted head to journal

When FileStore.compact() returns the journal.log does not necessarily contain the head created by the compactor. This can lead to problems downstream like e.g. in OAK-6894 where the compactor tool wrote the wrong (i.e. uncompacted) head to the journal.log.

Proposed fix is to call on of the FileStore.flush() methods after compaction and add a test case that verifies the journal.log contains the correct head state.