Kid restart leads to persistent queue overflows, delays/timeouts (#706)

Kid restart leads to persistent queue overflows, delays/timeouts (#706)

    WARNING: communication with ... may be too slow or disrupted...
    WARNING: abandoning ... I/Os
    ERROR: worker I/O push queue for ... overflow...
    ERROR: Collapsed forwarding queue overflow...

SMP queues rely on the shared memory QueueReader::popSignal flag to
reduce the number of UDS messages that queue readers and writers need to
send each other. If the flag is true but there is no corresponding "wake
up, you have new queued items to read" UDS message for the reader, the
reader may stall. This happens when the reader restarts (e.g., after
hitting an assertion) while the flag is true. A stalled queue reader
leads to delays and queue overflows:

* When the problem affects worker-disker queues, disk I/O delays under
  the hard-coded 7-second timeout are not reported to the admin but may
  affect user experience. Larger delays trigger level-1 WARNINGs. Push
  queue overflows trigger level-1 ERRORs.

* Transient worker-worker queue problems may stall concurrent
  transactions that are reading from the cache entry being written by
  another process. Overflows trigger level-1 ERRORs.

The restarted worker usually starts working just fine because it does
not expect any messages. A busy restarted worker may also appear to
continue working fine because workers always pop queued items before
pushing new ones -- as long as the worker queues new items, it will see
and pop responses to earlier requests, masking the problem. However, the
"stuck popSignal" problem never goes away: Squid only clears the flag
when receiving a notification, but sending new notifications is blocked
by that stuck flag.

Upon kid start, we now clear popSignal (to reflect the actual
communication state) and empty the queue (to reduce overflows). Since
commit 4c21861, any corresponding in-flight UDS queue notification is
ignored because it was sent to the previous process playing the same kid
role. The queue writer will see the false popSignal flag and send a new
notification when queuing a new item, preventing queue stalls.

Also properly ignore stale disker responses about cache_dirs we have not
opened yet, especially since we are now trying to empty the queues ASAP
after a restart, before Coordinator has a chance to inform us about
available diskers, populating the IpcIoFiles container. We already have
similar protection from stale UDS messages and from stale disker queue
messages about _opened_ cache_dirs ("LATE disker response to...").

Also report SMP queue flags in mgr:store_queues.