Store Open ForestDB Snapshots

Open the ForestDB Snapshots before storing in
snapshot container. A snapshot can be opened after commit and
before any other DB operation(insert/delete etc). So a snapshot
is now opened after commit and stored.

Change-Id: Ia8a5367fb4a011f41c5fc3253c67f8a07cf76080
Reviewed-on: http://review.couchbase.org/39597
Reviewed-by: John Liang <johliang00@gmail.com>
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>