Remove throttled worker spawn feature

Controlling the number of keygen workers provides enough tuning
flexibility especially with the ability to block until a set size is
achieved.

The throttling design results in continuous timeout message overhead
when set to zero.

Refactoring will remove the need for a scheduled timeout value
entirely.
Move async_refill call from msg receive to DOWN handling

We'll always receive a DOWN from spawned workers so better to trigger
the refill from there than from the receive of the key or timeout.
Remove keygen_cache_pause feature

Throttling the adjusting number of workers is enough. Extra complexity
is not needed.
Update docs