Tx timestamp computation

[raduchis]

Reasoning behind the pull request

• timestamp from api transactions is not computed correctly after supernova activation

Proposed changes

• use roundhandler to compute timestamp and timestampMs

Testing procedure

• after the supernova activation check that api transactions timestamps are not in the future

Pre-requisites

Based on the Contributing Guidelines the PR author and the reviewers must check the following requirements are met:

• was the PR targeted to the correct branch?
• if this is a larger feature that probably needs more than one PR, is there a feat branch created?
• if this is a feat branch merging, do all satellite projects have a proper tag inside go.mod?

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 77.62%. Comparing base (53da698) to head (4126cf7).
⚠️ Report is 5 commits behind head on feat/testnet-fixes.

Files with missing lines	Patch %	Lines
process/block/metablock.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##           feat/testnet-fixes    #7846   +/-   ##
===================================================
  Coverage               77.62%   77.62%           
===================================================
  Files                     885      885           
  Lines                  125425   125414   -11     
===================================================
- Hits                    97357    97353    -4     
+ Misses                  21585    21579    -6     
+ Partials                 6483     6482    -1     

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