ACCUMULO-2579 Fix a bug where close was being written to replication table prematurely

A minc does not equate to a WAL no longer being used. The data up to the present length
in the WAL is ready to be replicated, but other tablets may still reference it. We can
only determine that a wal is "closed" when there are no additional references to it
in accumulo.metadata or when recovery occurs.
ACCUMULO-2582 Add some tracing to the work maker
ACCUMULO-2575 Add another step to the GC to remove references to old WALs

The only that we know a WAL is fully closed is when no references in the metadata
table exist for that file. When this happens, we're safe to update the replication
record from open->closed. This allows GC of those files to proceed normally.