Revised heartbeat check

[computezrmle]

Volunteers from LHC@home often report issues (false positives) in connection with the heartbeat file detection.
In most cases it affects Windows hosts.

This PR introduces a revised heartbeat file handling and a slightly modified reporting.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 9.83%. Comparing base (d344bf9) to head (b7f2616).
Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##             master   #5864      +/-   ##
===========================================
- Coverage     10.76%   9.83%   -0.94%     
  Complexity     1068    1068              
===========================================
  Files           280     280              
  Lines         36494   36310     -184     
  Branches       8444    8444              
===========================================
- Hits           3930    3571     -359     
- Misses        32175   32196      +21     
- Partials        389     543     +154     

see 12 files with indirect coverage changes