op-acceptance-tests: interop: misc improvements to load test

[joshklop]

• Properly protect the rand.Source in initiators
• Spam initiating messages in parallel
• Retry submission forever for valid messages

[codecov]

Codecov Report

Attention: Patch coverage is 68.42105% with 12 lines in your changes missing coverage. Please review.

Project coverage is 43.97%. Comparing base (a770f20) to head (855f656).
Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
op-service/txplan/txplan.go	0.00%	7 Missing ⚠️
...ceptance-tests/tests/interop/loadtest/initiator.go	75.00%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #16101      +/-   ##
===========================================
- Coverage    46.97%   43.97%   -3.00%     
===========================================
  Files         1368     1207     -161     
  Lines       110009   101228    -8781     
===========================================
- Hits         51678    44518    -7160     
+ Misses       54684    53197    -1487     
+ Partials      3647     3513     -134     

Flag	Coverage Δ
cannon-go-tests-64	?
contracts-bedrock-tests	?

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

Files with missing lines	Coverage Δ
...acceptance-tests/tests/interop/loadtest/helpers.go	100.00% <100.00%> (ø)
...acceptance-tests/tests/interop/loadtest/relayer.go	93.75% <100.00%> (+0.41%)	⬆️
...ceptance-tests/tests/interop/loadtest/initiator.go	86.00% <75.00%> (-5.43%)	⬇️
op-service/txplan/txplan.go	27.95% <0.00%> (-0.63%)	⬇️

... and 165 files with indirect coverage changes

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