accounts-db: Improve sharding of the read-only cache

The default amount of shards in `DashMap` is `num_cpus * 4`. That means
256 on validators with 64 threads. A number of accounts held in caches
on mainnet beta validators with default configuration is around 44-56K,
which results in around 200 accounts per shard. That means, locking a
shard locks an access to 200 accounts.

Fix that by increasing the amount of shards to 65536, which in the best
case  will keep just one account per shard. Acquiring a lock shouldn't
lock access to any other accounts.

A single `RwLock<RawTable<(K, V)>>`, without the key and value included,
takes 48 bytes. Therefore, 65536 shards are taking 3145728 B, which
rounds up to 3MB. It's an acceptable increase in memopry usage.

accounts-db/src/read_only_accounts_cache.rs

@@ -31,6 +31,11 @@ use {
 const CACHE_ENTRY_SIZE: usize =
     std::mem::size_of::<ReadOnlyAccountCacheEntry>() + 2 * std::mem::size_of::<ReadOnlyCacheKey>();
 
+/// Number of cache shards. This number is close to the number of accounts
+/// in cache observed on mainnet beta validators, therefore it should result
+/// in each accout having its own shard and reduced amount of locks.
+const SHARDS: usize = 65536;
+
 type ReadOnlyCacheKey = Pubkey;
 
 #[derive(Debug)]
@@ -98,7 +103,7 @@ impl ReadOnlyAccountsCache {
     ) -> Self {
         assert!(max_data_size_lo <= max_data_size_hi);
         assert!(evict_sample_size > 0);
-        let cache = Arc::new(DashMap::default());
+        let cache = Arc::new(DashMap::with_shard_amount(SHARDS));
         let data_size = Arc::new(AtomicUsize::default());
         let stats = Arc::new(AtomicReadOnlyCacheStats::default());
         let evictor_exit_flag = Arc::new(AtomicBool::new(false));