Feature/expose monitor socket for active poller

[dietelTiMaMi]

Motivation:
zmq::monitor_t in it's current form is inconvenient to use in combination with zmq::active_poller_t
the issue is, that the socket isn't available to the user of zmq::monitor_t

Proposal:
split zmq::monitor_t::check_event() to move the actual reading and processing of the events into a new protected method zmq::monitor_t::process_event()
provide a protected socket_ref for using the monitor_socket for polling in a derived class.

Execution:
The proposed changes were done tozmq.hpp
A test case demonstrating the desired usage was added to tests/monitor.cpp

By inheriting from zmq::monitor_t you can now implement custom ways of polling for socket events.
The modified api is hidden for "rookie" users that might accidentaly call process_event() without polling for changes before use.

[dietelTiMaMi]

Please restart the check, ran on my fork aftrer removing the chrono literals.
they are not available in C++11.
It seems the Ubuntu 18 runners are no longer existent

[dietelTiMaMi]

@gummif Could you please relaunch the CI tests?
in the first run there was some C++14 code that didn't compile in C++11 inside the test
that is now fixed.

I also ran the CI in my fork and except for the Ubuntu 18.04 runners that are no longer existent everything went through.

[dietelTiMaMi]

@gummif

Thanks for restarting the workflow.
Do you see any problems for merging this?
If you would like to have any changes, I will gladly contribute to be able to use it in our project.

[gummif]

Hi I will try to take a look today.

[gummif]

You decide if the comment is worth fixing, otherwise looks good.

[coveralls]

Pull Request Test Coverage Report for Build 6106326776

• 35 of 70 (50.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.7%) to 86.965%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
zmq.hpp	35	70	50.0%

Totals
Change from base Build 5678621379:	0.7%
Covered Lines:	854
Relevant Lines:	982

---

💛 - Coveralls