Pick an RPC node at random to avoid getting stuck on a bad RPC node (bp #7759)

[mergify]

This is an automated backport of pull request #7759 done by Mergify.io

Cherry-pick of ad4d41e has failed:

On branch mergify/bp/v0.21/pr-7759
Your branch is up to date with 'origin/v0.21'.

You are currently cherry-picking commit ad4d41e60.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:

	modified:   validator/src/main.rs

Unmerged paths:
  (use "git add <file>..." to mark resolution)

	both modified:   Cargo.lock
	both modified:   validator/Cargo.toml

To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

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[codecov]

Codecov Report

Merging #7762 into v0.21 will increase coverage by 22.6%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           v0.21   #7762      +/-   ##
========================================
+ Coverage   56.9%   79.6%   +22.6%     
========================================
  Files        239     239              
  Lines      65100   46558   -18542     
========================================
  Hits       37083   37083              
+ Misses     28017    9475   -18542