Optimize max_flushed_root update in flushing roots

[HaoranYi]

Problem

When updating max flushed root in accounts index, we don't need to update it for ever flushed root in the loop. We can update it with the last flushed root.

split from #4304

Summary of Changes

update max flushed root in account index with the last flushed root.

Fixes #

[HaoranYi]

rename: cached_roots -> flushed_roots for clarity.

[HaoranYi]

probably won't matter much in actual performance. But it is still nice to have.

[brooksprumo]

One thing I thought of, where dooes the accounts_cache's max_flush_root get used? Anywhere in the foreground/transaction processing? That's the only place I can think of where delaying when set_max_flush_root() may make a difference.

[HaoranYi]

One thing I thought of, where dooes the accounts_cache's max_flush_root get used? Anywhere in the foreground/transaction processing? That's the only place I can think of where delaying when set_max_flush_root() may make a difference.

There are 3 usages of max_flushed_root.

1. add_root()
2. flush_accounts_cache()
3. handle_snapshot_request()

2 and 3 are from the same account background thread. So this change should not
affect them.

1 is used from bank thread.

We can prove that any call of slot for add_root() will be greater than max_flushed_root.

Proof:

1. root will be strictly increasing when we call add_root().
2. max_flushed_root is computed from the split from w_maybe_flushed_roots
3. 'w_maybe_flushed_roots' is populated by add_root().

Claim: Any slot 'S' that called on add_root() must be greater than the max slot in w_maybe_flushed_roots.
This is because 'w_maybe_flushed_roots' only contains previous slots, which are less than current slot 'S'.

Claim: max_flushed_root must be less than max slot in w_maybe_flushed_roots because of (2).

Therefore, S > max('w_maybe_flushed_roots') >= max_flushed_root.

Q.E.D

Therefore, 1 won't be affected by this change too.