[Epoch Sync] Verify merkle proofs for the first block in current epoch.

[robin-near]

• Verify the first block of the current epoch against the last final block of the current epoch, using the merkle proof.
• Verify the last and second last headers before the first header.
• Verify the partial merkle tree (note: this is NOT the merkle proof!) of the first block; add is_well_formed() to facilitate that validation.
• Clearly document what every field of EpochSyncProof is validated against.

[codecov]

Codecov Report

Attention: Patch coverage is 74.50980% with 13 lines in your changes missing coverage. Please review.

Project coverage is 71.21%. Comparing base (244422d) to head (d1a8885).
Report is 19 commits behind head on master.

Files with missing lines	Patch %	Lines
chain/client/src/sync/epoch.rs	66.66%	12 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12330      +/-   ##
==========================================
- Coverage   71.25%   71.21%   -0.05%     
==========================================
  Files         838      839       +1     
  Lines      169020   169777     +757     
  Branches   169020   169777     +757     
==========================================
+ Hits       120442   120908     +466     
- Misses      43338    43615     +277     
- Partials     5240     5254      +14     

Flag	Coverage Δ
backward-compatibility	0.16% <0.00%> (-0.01%)	⬇️
db-migration	0.16% <0.00%> (-0.01%)	⬇️
genesis-check	1.22% <0.00%> (-0.01%)	⬇️
integration-tests	39.01% <56.86%> (-0.04%)	⬇️
linux	70.65% <74.50%> (-0.05%)	⬇️
linux-nightly	70.79% <74.50%> (-0.06%)	⬇️
macos	50.41% <23.52%> (+0.03%)	⬆️
pytests	1.53% <0.00%> (-0.01%)	⬇️
sanity-checks	1.34% <0.00%> (-0.01%)	⬇️
unittests	64.27% <23.52%> (+0.06%)	⬆️
upgradability	0.21% <0.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[staffik]

LGTM!
Could you remind what happens if last final block of current epoch has not been endorsed by next epoch's block producers?

[robin-near]

@staffik it doesn't matter; as long as the current epoch's block producers endorsed it, it is valid.

[robin-near]

@Longarithm @shreyan-gupta could I please get a review on this - thank you!!

[shreyan-gupta]

Approving to unblock. Took a high level look and lgtm, however would be nice for someone to take a more detailed look.