MB-26157: Don't modify the fetched item
MB-26157: Don't modify the fetched item

Other threads may operate on the same object while we're trying
to prune it (and we could fail storing the object back with an
invalid CAS).

Change-Id: Ic37317b4efe38bed3fa6212a335c6dfa94910722
Reviewed-on: http://review.couchbase.org/83671
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Manu Dhundi <158873d90a7ef40f3637a222b7329c09d0222554@couchbase.com>
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>
Well-Formed: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine