Log when node up/down events occur
Log when node up/down events occur

This is a cherry-pick of:

https://github.com/cloudant/couch_replicator/commit/d40bb6f2e603a7c81f777cc1c4c200ad34c3db42

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