Fix replicator checkpoint issue by making sure checkpoint doc ID stays constant
Fix replicator checkpoint issue by making sure checkpoint doc ID stays constant

Memoize the value returned by .remoteCheckpointDocID so it stays the same even
if a setting changes — namely, the -stop method sets _continuous=NO for some reason.
Fixes #301

ryuzmukhametov/couchbase-lite-ios,brendand/couchbase-lite-ios,pegli/couchbase-lite-ios,ipmobiletech/couchbase-lite-ios,macoscope/couchbase-lite-ios,lordkev/couchbase-lite-ios,jkingyens/couchbase-lite-ios,couchbase/couchbase-lite-ios,ryuzmukhametov/couchbase-lite-ios,macoscope/couchbase-lite-ios,lordkev/couchbase-lite-ios,ipmobiletech/couchbase-lite-ios,pellekrogholt/couchbase-lite-ios,brendand/couchbase-lite-ios,pegli/couchbase-lite-ios,amco/couchbase-lite-ios,pellekrogholt/couchbase-lite-ios,mz2/couchbase-lite-ios,alfared/couchbase-lite-ios,amco/couchbase-lite-ios,pegli/couchbase-lite-ios,amco/couchbase-lite-ios,pegli/couchbase-lite-ios,tomco/couchbase-lite-ios,couchbase/couchbase-lite-ios,macoscope/couchbase-lite-ios,gotmyjobs/couchbase-lite-ios,pellekrogholt/couchbase-lite-ios,gotmyjobs/couchbase-lite-ios,Chavenay/couchbase-lite-ios,tomco/couchbase-lite-ios,amco/couchbase-lite-ios,alfared/couchbase-lite-ios,mz2/couchbase-lite-ios,amco/couchbase-lite-ios,macoscope/couchbase-lite-ios,couchbase/couchbase-lite-ios,ryuzmukhametov/couchbase-lite-ios,Chavenay/couchbase-lite-ios,brendand/couchbase-lite-ios,Chavenay/couchbase-lite-ios,ipmobiletech/couchbase-lite-ios,pellekrogholt/couchbase-lite-ios,lordkev/couchbase-lite-ios,ryuzmukhametov/couchbase-lite-ios,ipmobiletech/couchbase-lite-ios,brendand/couchbase-lite-ios,amco/couchbase-lite-ios,gotmyjobs/couchbase-lite-ios,couchbase/couchbase-lite-ios,lordkev/couchbase-lite-ios,jkingyens/couchbase-lite-ios,tomco/couchbase-lite-ios,gotmyjobs/couchbase-lite-ios,mz2/couchbase-lite-ios,couchbase/couchbase-lite-ios,jkingyens/couchbase-lite-ios,couchbase/couchbase-lite-ios,alfared/couchbase-lite-ios,jkingyens/couchbase-lite-ios,amco/couchbase-lite-ios,tomco/couchbase-lite-ios,alfared/couchbase-lite-ios,brendand/couchbase-lite-ios