Cleanup item handling code in store.c

Change-Id: I428373fa5eb26945f9ffc3ce843d5bedab6d8993
Reviewed-on: http://review.couchbase.org/27918
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>