Uses dashmap for cache_for_accounts_lt_hash

[brooksprumo]

Problem

The accounts lt hash cache is a HashMap behind a single RwLock. When inserting into the cache, we do a lookup with a read lock, then—if not present—do an insert with a write lock. This can have contention with other replay threads also trying to access/update the cache.

Summary of Changes

Replace the cache's RwLock + HashMap with a DashMap.

Results

I ran this PR vs master (ish), both with the accounts lt hash cli arg enabled. Since upsert into the cache is part of tx processing, we want to reduce latency as much as possible. The upsert is two parts: first, lookup to see if present (read lock only), and second, if not present, insert (with write lock).

brux1 is this PR, brux2 is master.

for lookup, brux1 is red and brux2 is yellow

lookup mean: dashmap is faster on average

lookup max: dashamp has better worst-case lookup times

---

The speedup in lookup (above) is the main win. The time spent inserting is mostly the same.

Here's a graph of the sum of the total inspect time (lookup + insert), which also shows a speed up with this PR.

brux1 is blue, brux2 is purple.

[brooksprumo]

@HaoranYi - requesting your review as an accounts-db expert.
@alessandrod - requesting your review as a performance expert. I want to make sure this isn't misguided/shortsighted/etc!

[HaoranYi]

Nice. I like it.