core/consensus: change round AB test timers

[corverroos]

Refactors the roundtimer AB testing candidates: Just the legacy inc and the new eager_dlinear. This is after simulation benchmarking showed that the following timer would work best:

• eager, so start times are aligned between all nodes.
• double, doubles round times if pre-prepare is detected (extended round with valid leaders).
• linear, 1s, 2s, 3s, which means 4 round in 10s, which should be enough for almost all clusters.

category: refactor
ticket: #2117

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (fb6aa21) 53.68% compared to head (791557b) 53.69%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2213   +/-   ##
=======================================
  Coverage   53.68%   53.69%           
=======================================
  Files         186      186           
  Lines       24261    24248   -13     
=======================================
- Hits        13025    13020    -5     
+ Misses       9629     9622    -7     
+ Partials     1607     1606    -1     

Impacted Files	Coverage Δ
core/consensus/roundtimer.go	100.00% <100.00%> (+3.07%)	⬆️

... and 6 files with indirect coverage changes

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