CBIDXT-177 Drop Index Race Condition with Flush

If there is a flush in progress and drop index gets executed,
the forestdb files can get removed while being written to.
To prevent this, indexer needs to wait for in progress flush
of that bucket to finish for which index is being dropped.

Change-Id: Id7de47066a1806e2834ad982a725b94765fc596b
Reviewed-on: http://review.couchbase.org/42750
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>