This is required due to lack of a graceful shutdown: there is a small gap
between the runtime's memory pool release and router process's exit. Thus, a
worker thread may start processing a request between these two operations,
which may result in an http fields hash access and subsequent crash.

To simplify issue reproduction, it makes sense to add a 2 sec sleep before
exit() in nxt_runtime_exit().

After 41331471eee7 completion handlers should complete next buffer in chain.
Otherwise buffer memory may leak.

Thanks to Peter Tkatchenko for reporing the issue and testing fixes.

An earlier attempt (ad6265786871) to resolve this condition on the
router's side added a new issue: the app could get a request before
acquiring a port.