Move native updater sort step to happen just before btree update

Currently record sorting for all btrees are performed one by one
and after that btree updates are performed in sequential order.
The disk buffer cache for sort files might get overwritten by
the time actual btree update happens and hence it has to pay
the cost of reading from disk again. By moving sort step just before
every btree update operation ensures better utilization of disk
buffer cache.

Change-Id: I5b7a720c4888e6b3af8ffc045ba3663fde892216
Reviewed-on: http://review.couchbase.org/33122
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>