MB-8292: Switch engine onto snaphot stats add_stat callback

The add_casted_stat function will switch the current engine off
of the thread so we need to switch it back on when entering the
snapshot stats add_stat callback.

Change-Id: I2208f37c0d028908a803176b2efabd867f8a06f3
Reviewed-on: http://review.couchbase.org/26811
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>