core/qbft: fix trimming issue

[corverroos]

Fixes buffer trimming issue:

• There is only a few uses for Justification messages:
  • a) Check if a received PRE-PREPARE is justified (only use justifications of that single message)
  • b) Check if a received ROUND-CHANGE is justified (only use justifications of that single message)
  • c) Generate justification of a quorum ROUND-CHANGEs (select from all buffered messages and their justifications)
• We only need previously received justifications for (c), and we can get that by flattening the buffer.
• Since we do not add old justifications to the buffer anymore
• We can trim all old messages again.

Also make code a bit more explicit and readable.
Also ensure that send-to-self in the test is always first and never drops or jitters.

category: bug
ticket: #445

[codecov]

Codecov Report

Merging #507 (7435c25) into main (051b60c) will decrease coverage by 0.17%.
The diff coverage is 44.11%.

@@            Coverage Diff             @@
##             main     #507      +/-   ##
==========================================
- Coverage   56.44%   56.26%   -0.18%     
==========================================
  Files          87       87              
  Lines        7989     8001      +12     
==========================================
- Hits         4509     4502       -7     
- Misses       2883     2898      +15     
- Partials      597      601       +4     

Impacted Files	Coverage Δ
core/qbft/qbft.go	65.53% <44.11%> (-2.97%)	⬇️
app/app.go	63.31% <0.00%> (-0.89%)	⬇️

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[corverroos]

no need to filter again, the input is already Frc

[xenowits]

LGTM!