mon/health_checks: associate a count with health_alert_t

mon/health_checks: associate a count with health_alert_t

0 means this is a singleton.  Otherwise, we can sum this up, either
via merge() or get_or_add().

We always structure this so the count goes toward zero (more healthy), so
if a value is too low, then we count how much too low it is.

Signed-off-by: Sage Weil <sage@redhat.com>