Rework `quiesced` mode to support writable databases

This patch changes the `quiesce` system to allow reopening the store in
writable mode, in order to support `defrag` on a quiesced database.

See: ARAKOON-442
See: http://jira.incubaid.com/browse/ARAKOON-442
Run `defrag` in quiesced mode

See: ARAKOON-442
See: http://jira.incubaid.com/browse/ARAKOON-442
CI: Use `camltc.999`
Run `defrag` in its own OS thread

See: ARAKOON-442
See: http://jira.incubaid.com/browse/ARAKOON-442
Clean-up `Sync_backend:quiesce_db`

Get rid of an otherwise unused `ref`.
Pass around `bool`s to `Store.reopen`
Merge remote-tracking branch 'origin/1.6' into ARAKOON-442-1.6

* origin/1.6:
  (manual) backport of 1.7's master_state changes

Conflicts:
	src/paxos/multi_paxos_fsm.ml
Merge pull request #252 from Incubaid/ARAKOON-442-1.6

ARAKOON-442: Allow slaves running `defrag` to keep participating in a cluster