add support for last-modified-date for persistent store objects, and use that to determine which mgmt nodes are live
address code review comments from brooklyncentral/brooklyn#1464 - mainly tidying up names and logging
comments about one bit of code spotted by @aledsage in code review
(failing) test for split-brain scenario
fixes multi-master problems by having one demote himself, with tests for demotion and for massively-concurrent startups also
This closes #12