AccountsHashVerifier purges old bank snapshots

[brooksprumo]

Problem

We retain more bank snapshots than we need, longer than we need.

Building on #31511, what if a validator turns off generating snapshots? In that case, the node will still take bank snapshots since they are needed for accounts hash verification. Only snapshots intended for archival are turned off (this is the majority though).

In this scenario, we may not clean up old bank snapshots.

This is/will especially be true once AccountsBackgroundService is not in charge of cleaning up old bank snapshots.

In that case, since AccountsHashVerifier is the last one to consume the bank snapshot, it should be the one to clean them up as well.

Summary of Changes

AccountsHashVerifier will purge bank snapshots older than the one that was just processed IFF snapshot (archival) is disabled.

[codecov]

Codecov Report

Merging #31519 (c17312a) into master (8e5e66f) will decrease coverage by 0.1%.
The diff coverage is 57.1%.

@@            Coverage Diff            @@
##           master   #31519     +/-   ##
=========================================
- Coverage    81.3%    81.2%   -0.1%     
=========================================
  Files         733      733             
  Lines      206693   206698      +5     
=========================================
- Hits       168064   168033     -31     
- Misses      38629    38665     +36     

[apfitzge]

lgtm - change seems reasonable; if we aren't dooing archiving, we don't need these snapshots except for accounts hashing or fast-boot.