userdbd: when we hit a flood of requests to start more workers, don't exit

userdbd: when we hit a flood of requests to start more workers, don't exit

Let's tweak what we do if we detect a flood of requests to start more
workers: if none of the workers ever sticks (i.e. the worker count is
zero) then let's just give up, as before.

Otherwise, let's just not start more workers for a while, and do so
again after a while. Thus spawning ofr workers will "cool off" for a
while.

Fixes: #27028