Removed support for “_conflicts” property in doc passed to map blocks.

It didn’t work quite right (could miss some conflicts) and taking it out speeds up
view indexing a little bit.
Developers who want to find conflicts should use an allDocs query with the
only-conflicts option instead.
Fixes #159
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
Safer handling of unexpected revisions returned from _bulk_get

Don’t process such a revision; it’ll cause an exception later.
For #300
Merge branch 'master' of https://github.com/couchbase/couchbase-lite-ios