Core: delete accept timer on worker shutdown

Core: delete accept timer on worker shutdown

Previously, when the worker file descriptor table was full, a listen
connection might be disabled and a timeout might be set for re-enabling
it.  If during this period the worker received the NGX_CMD_QUIT signal,
the accept connection would be recycled, but the timer would remain set.
On timer expiration, an attempt to accept a new connection would result
in "accept4() failed (9: Bad file descriptor)" alert because c->fd was
reset to -1.

Fix is to delete accept timer when closing listen connections.