This should result in the loop getting serviced more often when there
are many callbacks to run. This is hooked up to `getswitchinterval` to
allow control.

We check the interval every 50 callbacks. That's hardcoded, which I
don't like, but the 1000 callback number was hardcoded before, so it's
not really much of a change. A future extension would be to calculate
the check number dynamically based on dividing the switch interval.

I don't have a specific test case for this change because I'm a little
fried, but all the other tests pass. I did manually verify that the
example in #1071 matches expectations---that could probably be turned
into a test case with some work.

Fixes #1072
Fixes #1071
Fixes #1068