Fix stopLock race condition
Fix stopLock race condition

Per code review, fix stopLock race condition by always creating and nulling the lock object on the replicator thread.

brendand/couchbase-lite-ios,amco/couchbase-lite-ios,pellekrogholt/couchbase-lite-ios,brendand/couchbase-lite-ios,couchbase/couchbase-lite-ios,amco/couchbase-lite-ios,macoscope/couchbase-lite-ios,lordkev/couchbase-lite-ios,tomco/couchbase-lite-ios,pellekrogholt/couchbase-lite-ios,gotmyjobs/couchbase-lite-ios,ipmobiletech/couchbase-lite-ios,couchbase/couchbase-lite-ios,pegli/couchbase-lite-ios,couchbase/couchbase-lite-ios,alfared/couchbase-lite-ios,pegli/couchbase-lite-ios,alfared/couchbase-lite-ios,macoscope/couchbase-lite-ios,alfared/couchbase-lite-ios,couchbase/couchbase-lite-ios,pegli/couchbase-lite-ios,amco/couchbase-lite-ios,amco/couchbase-lite-ios,ipmobiletech/couchbase-lite-ios,brendand/couchbase-lite-ios,tomco/couchbase-lite-ios,amco/couchbase-lite-ios,brendand/couchbase-lite-ios,macoscope/couchbase-lite-ios,couchbase/couchbase-lite-ios,pegli/couchbase-lite-ios,lordkev/couchbase-lite-ios,gotmyjobs/couchbase-lite-ios,pellekrogholt/couchbase-lite-ios,tomco/couchbase-lite-ios,pellekrogholt/couchbase-lite-ios,amco/couchbase-lite-ios,ipmobiletech/couchbase-lite-ios,lordkev/couchbase-lite-ios,lordkev/couchbase-lite-ios,macoscope/couchbase-lite-ios,amco/couchbase-lite-ios,gotmyjobs/couchbase-lite-ios,ipmobiletech/couchbase-lite-ios,brendand/couchbase-lite-ios,tomco/couchbase-lite-ios,alfared/couchbase-lite-ios,couchbase/couchbase-lite-ios,gotmyjobs/couchbase-lite-ios