instead remove 'hub' from _threadlocal when greenlet dies, so it will be restored by get_hub()
also, pass exit code of event_dispatch to DispatchExit instead of text message

- also use queue.Queue instead of coros.Queue

- when notifying links, do not reschedule the rest of the links in case of
  an error; just print the traceback since that what the parent will do anyway

- now None (the default) means raise self, False means "silent" interrupt;
  True does not mean anything.

- add hex id in __repr__
- fix formatting bug in __repr__
- new __str__ that is split into several lines to make it more readable formatted that repr
- use __str__ when printng reports on exception raised by callbacks

although the feature is nice, it's did not work properly, e.g. for PriorityQueue.
neither the standard Queue nor coros.channel/queue do not support it and so are we now.

nicer error messages for asserts in wait_reader() and wait_writer()