Add back multi-sig support and fix the multi-sig check for harmony nodes

[rlan35]

For multi-sig, there was a condition to disallow multi-sig signed from multiple accounts. It is to prevent malicious people to double sign and merge with others signature to trigger slashing for other people.

This condition broke testnet since in testnet our nodes contain multiple keys from different accounts. This PR fixes the condition so Harmony nodes can sign blocks with multi-sig without problems.

Basically, the first commit is a revert of a revert, which is already reviewed. The second commit is the fix: eeb8e31

Tested on local net. Will deploy to testnet.

[JackyWYX]

Looks good to me!

[codecov]

Codecov Report

Merging #3337 into main will decrease coverage by 0.13%.
The diff coverage is 22.79%.

@@            Coverage Diff             @@
##             main    #3337      +/-   ##
==========================================
- Coverage   34.49%   34.36%   -0.14%     
==========================================
  Files         210      209       -1     
  Lines       24693    24906     +213     
==========================================
+ Hits         8519     8560      +41     
- Misses      15523    15691     +168     
- Partials      651      655       +4     

Impacted Files	Coverage Δ
consensus/checks.go	0.00% <0.00%> (ø)
consensus/consensus.go	59.09% <ø> (ø)
consensus/consensus_v2.go	0.00% <0.00%> (ø)
consensus/double_sign.go	0.00% <0.00%> (ø)
consensus/leader.go	0.00% <0.00%> (ø)
consensus/quorum/one-node-one-vote.go	0.00% <0.00%> (ø)
consensus/threshold.go	0.00% <0.00%> (ø)
consensus/validator.go	0.00% <0.00%> (ø)
consensus/view_change.go	7.75% <0.00%> (-0.36%)	⬇️
consensus/votepower/roster.go	45.29% <0.00%> (ø)
... and 11 more

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