Persistence hardening - creates copies, reports stats, and more


Key changes:
* hot backup (where a node will rebind, but not attempt to auto-promote, in case we want an external source to decide on promotion)
* metrics on rebind, persistence, etc, including errors via REST
* failure on too many dangling references
* REST API can export persisted state
* copies of persisted state are made on any promotion/demotion event

This completes the persistence-related improvements I've wanted to make.

The last two have been in this PR for a few days; the first three items (in the list above) are new.  This corresponds to the final 4 commits below.
Always defaults to local file system.