sched: improve infinite loop detection

sched: improve infinite loop detection

The "infinite loop in scheduling" fatal error was observed on a system
running out of memory. Presumably, the execution of the process slowed
down due to memory thrashing so much that the dispatching loop wasn't
able to break with a single server polled at a 16-second interval.

To allow recovery in such a case, require for the error more than
20 handled timeouts and a rate higher than 100 per second.

Reported-by: Jamie Gruener <jamie.gruener@biospatial.io>