MB-11193: use right shard-level kvstore in warmup

General use of getAuxUnderlying() ends up causing all the warmup
threads to share the same CouchKVStore instance. This can result
in corruption of shared datastructures like dbFileRevMap.
Use the appropriate shard level KVStore instance for separation.
Change-Id: Iccbfc44c50e6381890392f4b50f02d2d6c23ab28
Reviewed-on: http://review.couchbase.org/37510
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>