don't update completed changes count when _all_docs fails

More work needed here.  See comment.

https://github.com/couchbase/couchbase-lite-android/issues/398
Add issue number to comments

https://github.com/couchbase/couchbase-lite-android/issues/398
give names to threads to make debugging easier

https://github.com/couchbase/couchbase-lite-android/issues/398