Fix race in remove_unrooted_race and flush_slot_cache

[carllin]

Problem

test_cache_flush_remove_unrooted_race is flaky https://buildkite.com/solana-labs/solana/builds/50349#b8b9efb2-8ace-496e-b975-e40a2a2ca3f4, where this assertion fails:

solana/runtime/src/accounts_db.rs

Line 4215 in 395ee4e

assert!(self.accounts_cache.remove_slot(slot).is_some());

. This is because between these two checks,

solana/runtime/src/accounts_db.rs

Lines 4232 to 4233 in 395ee4e

	self.accounts_cache.slot_cache(slot).and_then(|slot_cache| {
	let is_being_purged = {

, remove_unrooted_slots() could have removed the slot from the cache

Summary of Changes

Fixup order of checking, introduce new test.

Fixes #

[lijunwangs]

Looks good to me

Pull request has been modified.

[codecov]

Codecov Report

Merging #18785 (133724f) into master (23d9fe5) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##           master   #18785   +/-   ##
=======================================
  Coverage    82.8%    82.8%           
=======================================
  Files         441      441           
  Lines      124988   125045   +57     
=======================================
+ Hits       103554   103614   +60     
+ Misses      21434    21431    -3