Make ActiveSupport::Cache::CouchbaseStore threadsafe.

Change-Id: Ic75f317d58186008fa80ce814264058c53f7d3bf
Reviewed-on: http://review.couchbase.org/25294
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>