Make the header timestamp respect time of generation

[zivkovicmilos]

Description

This PR makes sure the header timestamp respects the time of generation.

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Checklist

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually

Additional comments

Please post additional comments in this section if you have them, otherwise delete it

[0xAleksaOpacic]

LGTM, manually tested backward compatibility

[codecov]

Codecov Report

Merging #678 (0d81404) into develop (b5462e0) will increase coverage by 0.01%.
The diff coverage is 58.00%.

@@             Coverage Diff             @@
##           develop     #678      +/-   ##
===========================================
+ Coverage    48.65%   48.67%   +0.01%     
===========================================
  Files          112      112              
  Lines        15364    15357       -7     
===========================================
- Hits          7476     7475       -1     
+ Misses        7254     7247       -7     
- Partials       634      635       +1     

Impacted Files	Coverage Δ
blockchain/subscription.go	72.00% <ø> (ø)
blockchain/testing.go	59.29% <ø> (ø)
consensus/ibft/consensus.go	0.00% <ø> (ø)
consensus/ibft/consensus_backend.go	0.00% <0.00%> (ø)
consensus/ibft/ibft.go	4.00% <0.00%> (ø)
consensus/ibft/pos.go	0.00% <ø> (ø)
consensus/ibft/sign.go	64.58% <ø> (ø)
consensus/ibft/verifier.go	0.00% <0.00%> (ø)
contracts/staking/query.go	71.42% <ø> (ø)
jsonrpc/errors.go	41.66% <ø> (ø)
... and 25 more

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