Rework Accounts for fast squash, hashing state and checkpoint recovery. (#3613)

* accounts rewrite

* ignore grow tests

* skip duplicate roots

* allow for a root race

* logger

* accounts_index tests

* tests

* tests

runtime/src/lib.rs

@@ -1,4 +1,5 @@
 mod accounts;
+mod accounts_index;
 pub mod append_vec;
 pub mod bank;
 pub mod bank_client;