Improve hb/s calculation in check arc

[adjordjevic-TT]

Now we calculate heartbeat frequency for each device by sleeping for just 0.2s (heartbeat interval is 0.1s) which introduces high variance in result (result can vary between 5 and 15 in a healthy case). Simply increasing sleep time would significantly increase execution time in setups with many devices (ex. galaxy).

Approach we can take is getting heartbeat values from all devices (with a timestamp) then sleep for some time (note we are now sleeping only once and not for every device) then we can collect heartbeat values again and calculate heartbeat frequency which much better precision and therefore can shrink the range we consider valid heartbeat frequency values.

[tt-vjovanovic]

If this is tt-triage related, what you can do is split script into two. First script will collect heart beat. Second script should have lower priority and it should be executed after all slow scripts. Then, we can have very precise heartbeat :)