🔊[RUMF-257] rework logging

[bcaudan]

• remove unneeded info
• ease logs search/reading
• track load event with a listener to compare with other timing sources that can be updated

[codecov-io]

Codecov Report

Merging #261 into master will increase coverage by 0.39%.
The diff coverage is 63.63%.

@@            Coverage Diff             @@
##           master     #261      +/-   ##
==========================================
+ Coverage   85.39%   85.79%   +0.39%     
==========================================
  Files          24       24              
  Lines        1315     1316       +1     
  Branches      277      275       -2     
==========================================
+ Hits         1123     1129       +6     
+ Misses        192      187       -5

Impacted Files	Coverage Δ
packages/rum/src/rum.ts	81.92% <0%> (ø)	⬆️
packages/rum/src/viewTracker.ts	90.54% <70%> (-3.99%)	⬇️
packages/rum/src/rum.entry.ts	71.42% <0%> (+2.04%)	⬆️
packages/core/src/requestCollection.ts	96.15% <0%> (+13.46%)	⬆️

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