Close All Open File Handles on Drop Index

When an index is dropped, all open forestdb handles
for slice and snapshots need to be explicitly closed.

Change-Id: I1f40d2eca6ae86781c22a0943d87f263dcef76dd
Reviewed-on: http://review.couchbase.org/42706
Reviewed-by: John Liang <johliang00@gmail.com>
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>