add callbacks to pulling end of pipe

[andre-merzky]

This PR adds callbacks to the pull end of a pipe. That is used in RP to efficiently implement component startup checks.

The PR also cleans up some log messages and improves thread termination.

[codecov]

Codecov Report

Attention: Patch coverage is 64.78873% with 25 lines in your changes missing coverage. Please review.

Project coverage is 61.58%. Comparing base (2f53b82) to head (c405616).
Report is 7 commits behind head on devel.

Files with missing lines	Patch %	Lines
src/radical/utils/zmq/pipe.py	71.11%	13 Missing ⚠️
src/radical/utils/zmq/pubsub.py	61.11%	7 Missing ⚠️
src/radical/utils/debug.py	0.00%	3 Missing ⚠️
src/radical/utils/zmq/queue.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            devel     #428      +/-   ##
==========================================
+ Coverage   61.56%   61.58%   +0.02%     
==========================================
  Files          62       62              
  Lines        6980     7015      +35     
==========================================
+ Hits         4297     4320      +23     
- Misses       2683     2695      +12     

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[andre-merzky]

This is ready for review now. Thanks!

[AymenFJA]

For the poller: For the PUSH mode, we don't need a Poller since we are only sending messages (and the socket doesn't need to wait for incoming messages), correct?

So maybe something like this:

    if mode == MODE_PUSH:
        self._connect_push(url)
        self._poller = None 

    elif mode == MODE_PULL:
        self._connect_pull(url)
        self._poller = zmq.Poller() 

[AymenFJA]

@andre-merzky, did you see this comment? I am unsure if you agree on this (it is a minor change). Besides this, the PR looks good to me.