First crack at rational health monitoring.

TODO:

* Rb lock monitoring
* Separate PLL and FLL lock
* PLL based on configurable tolerance
* FLL based on minimum history length
* If PLL locked but FLL unlocked, we should be able to give time but not enter holdover
Remove unused var
Attempt at improved PLL / FLL / holdover logic
Make health_get_ref_age actually return the age!
Not quite on the "coming out of holdover" bit.
Merge branch 'health'