Add a data_pending() method to Receiver

[eflumerf]

Description

Implement method required by DUNE-DAQ/iomanager#129, via the ZMQ_EVENTS ZeroMQ socket option.

Type of change

• New feature or enhancement (non-breaking change which adds functionality)

Testing checklist

• Unit tests pass (e.g. dbt-build --unittest)
• Minimal system quicktest passes (pytest -s minimal_system_quick_test.py)
• Full set of integration tests pass (daqsystemtest_integtest_bundle.sh)

This change should be tested together with DUNE-DAQ/iomanager#129. N.B that we should also clone dfmodules, fdreadoutlibs, fdreadoutmodules, trigger, and hsilibs into our software areas when testing these changes.

Further checks

• Code is commented where needed, particularly in hard-to-understand areas

[bieryAtFnal]

@eflumerf , I needed to tweak a couple of lines in these changes as a result of the most recent merge from develop. Hopefully, these changes look reasonable...

[bieryAtFnal]

These changes look good to me. I've tested them in conjunction with DUNE-DAQ/dfmodules#476, and they pass regression tests and are successfully used in draining the DataWriter input queue in the dfmodules changes.