Reload HA configuration when policy is changed.

After policy is changed make sure that HA configuration is reloaded.
This is needed by the cf-serverd as we don't want to restart it all
the time HA configuration is changed.
* I suspect this should happen *after* KeepPromises() ?  Surely that's what ensures assorted variables get updated.
* In any case, this is happening inside some locked state; it may be better to do this after we return from CheckFileChanges(), after those locks have been released.
If you do this as a follow-up to #1927, you can put it in the PolicyUpdateIfSafe() it introduces, similar to how I'm handling changes to call-collect interval.