mon: refactor health check map through PaxosMap

mon: refactor health check map through PaxosMap

This was motivated by confusing persistence of some health warnings during
testing of health warnings for cephx upgrades. Some services are only doing
health checks during ::encode_pending and others during ::tick. Make it
consistent.

Signed-off-by: Patrick Donnelly <pdonnell@ibm.com>
(cherry picked from commit 5e0c2b7aa74e378dadbaccb3584f750485b17e44)