Replace auxUnderlying with shard-level instance

Without this change we run the risk of race conditions
where threads that are supposed to work on their own shard
level CouchKVStore instance end up working on the same
auxUnderlying instance.

Makes read operations like warmup about 10% faster!

Change-Id: Iaa171edc00eccfbbe92ac29c7b41f2e89316d6d4
Reviewed-on: http://review.couchbase.org/37549
Reviewed-by: David Liao <david.liao@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>