Speed up mock instantiation.

Rather than creating a new instance for each time we reset the mock, we
make the instance persist throughout the lifetime of MockEnvironment

Change-Id: I3e63200df47f13cf43bc7bb340d5d55809768e2a
Reviewed-on: http://review.couchbase.org/34017
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>