MB-37772: Fix crash caused by initScanContext changes
MB-37772: Fix crash caused by initScanContext changes

Patch aaee19e5 breaks warmup because it re-uses a moved
unique_ptr. Fixed by recreating the object for each
call to initScanContext.

Change-Id: Iadd6a514174a3f3f6b7d56536d75fd2e690155d3
Reviewed-on: http://review.couchbase.org/123244
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>
Reviewed-by: Paolo Cocchi <9dd88c9f3e5cbab6e19a6c020f107e0c648ac956@couchbase.com>
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

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