mon: refactor osd failure report tracking

mon: refactor osd failure report tracking

- use structs to track allegedly failed nodes, and reports against them.
- use methods to handle report, and failure threshold logic.
- calculate failed_since based on OSD's reported failed_for duration

This will make it simpler to extend the logic when we add dynamic
grace periods.

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