v1.18: banking_stage: track VoteStorage size instead of costly iterator (backport of #743)

[mergify]

LatestUnprocessedVotes uses a costly lock grabbing iterator to compute length, which is used in various parts of the banking stage.
Instead track this separately in an atomic

---

This is an automatic backport of pull request #743 done by Mergify.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.5%. Comparing base (7b8ed3b) to head (7480c8d).
Report is 2 commits behind head on v1.18.

Additional details and impacted files

@@            Coverage Diff            @@
##            v1.18     #755     +/-   ##
=========================================
- Coverage    81.5%    81.5%   -0.1%     
=========================================
  Files         827      827             
  Lines      224833   224859     +26     
=========================================
- Hits       183453   183448      -5     
- Misses      41380    41411     +31     

[t-nelson]

pls hold this one

[alessandrod]

I support backporting this to 1.18 - with the caveat that I haven't reviewed the master PR and would like to take a look at this next week.

[t-nelson]

do we want to resurrect bp consideration here?

[apfitzge]

Stated on another PR: #1762 (comment)

If this PR is merged, we should also merge #1838 which fixes a race condition that was introduced with the change in #743

[willhickey]

Closing this as stale. If you think this is relevant please re-open and let @anza-xyz/backport-reviewers know why it is important