Accounts index opt (#4621)

* Add accounts_index bench

* Don't take the accounts index lock unless needed

* Accounts_index remove insert return vec and add capacity stats

* Use hashbrown hashmap for accounts_index

runtime/src/lib.rs

@@ -1,6 +1,6 @@
 mod accounts;
 pub mod accounts_db;
-mod accounts_index;
+pub mod accounts_index;
 pub mod append_vec;
 pub mod bank;
 pub mod bank_client;