MB-8903: Add failover log request

It's now possible to get the failover log with couch_upr.

Change-Id: I9b4868123babea93334a93dca447970d48e5ea35
Reviewed-on: http://review.couchbase.org/30543
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>