Ticket 5338: Race conditions in key rotation

[Nicolas Williams]

Also, a design where you have:

 - worker threads that never block (because async)
 - helper threads for event dispatching

allows you to match worker thread counts to CPU counts, and so keep
contention to a minimum.

Whereas simply multi-threading the KDC means that sometimes you may have
many threads on the run queues.

How big a deal this is depends on how much time will be spent blocking
on I/O while processing any one request.