Fix replicator manager `stop` change feed callback
Fix replicator manager `stop` change feed callback

```
changes_reader_cb({stop, EndSeq, _Pending}, ...) ->
   ...
```

at one point used to handle changes from `fabric:changes`. It was later
optimized to use shard change feeds, but shard change feed callbacks don't get
pending info with the `stop` message.

As a result replicator manager would always rescan all the changes in a shard
on any new change.

For reference, where `couch_changes.erl` calls the callback:
 https://github.com/apache/couchdb-couch/blob/master/src/couch_changes.erl#L654

Jira: COUCHDB-3104

fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb