listener: switch to level-triggered epoll

listener: switch to level-triggered epoll

On second thought, the ->requeue + accept retry code path isn't
worth the userspace complexity and overhead.  Level-triggered
epoll has always annoyed me since it takes an inefficient code
path in the kernel; but taking our less-efficient code path in
Perl seems even worse.  We also need to take load distribution
into account for multi-worker systems.