Microfiber 16.03

The Microfiber CouchDB replicator has been refined and a number of missing missing unit tests were added:

http://bazaar.launchpad.net/~microfiber/microfiber/trunk/view/head:/microfiber/replicator.py

Although the replicator isn't yet considered part of the stable API and might still undergo breaking API changes, the implementation is mature and is well proven under real-world use.

Some key changes in logging and behaviour:

* `replicate_one_batch()` now logs the number of documents that were missing on the dst_node for this particular batch, making it clearer when documents actually needed to be replicated vs when for that batch all of some of the documents were already present on the dst_node; logging wise, this makes the overall replication behaviour easier to understand when doing multi-master, bi-directional replication

* Although `replicate_one_batch()` still replicates in batches of 50 documents, `save_session()` now only saves a new replication checkpoint when the update_seq has advanced by at least 200 since the last checkpoint; especially when doing multi-master, bi-directional replication, this helps reduce the load on CouchDB because `dst.post(None, '_ensure_full_commit')` only needs to be called every 200 updates, whereas previously `replicate()` would save a checkpoint every 50 updates (whether or not new documents were missing on the dst_node)

* `replicate_continuously()` now likewise uses `save_session()` to checkpoint the replication every 200 updates, which means fewer changes need to be re-processed when a replication session is restarted and `replicate()` steps through the changes backlog