Squash the new working bank to ensure zero-balance accounts get purged

[mvines]

test_zero_balance_after_nonzero is green at the tip only because it completes before the first rotation occurs.

1. Force a rotation during test_zero_balance_after_nonzero to show it fails
2. Add a squash() into replay stage to avoid hitting

   solana/runtime/src/accounts.rs

   Lines 101 to 102 in ec35c1f

   	// store default account if balance is 0 and there's a checkpoint
   	self.accounts.insert(pubkey.clone(), Account::default());

[codecov]

Codecov Report

Merging #2950 into master will increase coverage by 2.1%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #2950     +/-   ##
========================================
+ Coverage    76.2%   78.3%   +2.1%     
========================================
  Files         127     127             
  Lines       20191   19641    -550     
========================================
  Hits        15391   15391             
+ Misses       4800    4250    -550