core/scheduler: subtract half rtt from sync offset

[corverroos]

Improve sync offset by subtracting the time it takes to send messages over the wire (rtt/2).

category: misc
ticket: #764
feature_flag: beacon_clock_sync

[codecov]

Codecov Report

Merging #779 (999e754) into main (e672e33) will decrease coverage by 0.11%.
The diff coverage is 36.66%.

@@            Coverage Diff             @@
##             main     #779      +/-   ##
==========================================
- Coverage   54.47%   54.35%   -0.12%     
==========================================
  Files         109      109              
  Lines       10667    10696      +29     
==========================================
+ Hits         5811     5814       +3     
- Misses       3992     4014      +22     
- Partials      864      868       +4     

Impacted Files	Coverage Δ
core/scheduler/metrics.go	100.00% <ø> (ø)
core/scheduler/clocksync.go	62.33% <29.62%> (-17.67%)	⬇️
core/scheduler/scheduler.go	71.69% <100.00%> (+0.15%)	⬆️
core/qbft/qbft.go	80.47% <0.00%> (-1.51%)	⬇️

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