Refactor snapshot daemon loop

* Fix buggy math calculation and simplifies randomization logic
  introduced by gh-732. The primary difference is that the new
  implementation uses fixed periods (=box.cfg.snapshot_period)
  shifted by a random offset instead of floating periods
  (=box.cfg.snapshot_period + random). In addition, snapshot daemon
  now never start snapshoting earlier than box.cfg.snapshot_period time.

* Use fiber.channel() to control daemon

Fixes #1912
See also #732