fix replication manager checkpointing (COUCHDB-3104)

Comprised of the following two commits in couch_replicator;

commit f67ebff225d63d20ccbcbd33de800f86911a5e79
Author: Nick Vatamaniuc <vatamane@gmail.com>
Date:   Mon Aug 15 03:20:44 2016 -0400

    Fix passing epoch in correctly with rep_db_checkpoint message.

    This bug was hidden previously because this code never ran due to
    another bug in handling the stop callback message from change feed.

    Jira: COUCHDB-3104

commit 40a087630b9b6688fe2db87f1e8adc7972c4a0c9
Author: Nick Vatamaniuc <vatamane@gmail.com>
Date:   Mon Aug 15 02:50:56 2016 -0400

    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

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