Make key value storage abstractions more accurate (#1267)

* Layer do_atomically() abstractions properly

* Reduce allocs and DRY get_key_for_col()

* Parameterize HotColdDB with hot and cold item stores

* -impl Store for MemoryStore

* Replace Store uses with HotColdDB

* Ditch Store trait

* cargo fmt

* Style fix

* Readd missing dep that broke the build

beacon_node/beacon_chain/tests/persistence_tests.rs

@@ -9,7 +9,7 @@ use beacon_chain::{
 };
 use sloggers::{null::NullLoggerBuilder, Build};
 use std::sync::Arc;
-use store::{HotColdDB, StoreConfig};
+use store::{HotColdDB, LevelDB, StoreConfig};
 use tempfile::{tempdir, TempDir};
 use types::{EthSpec, Keypair, MinimalEthSpec};
 
@@ -23,7 +23,7 @@ lazy_static! {
     static ref KEYPAIRS: Vec<Keypair> = types::test_utils::generate_deterministic_keypairs(VALIDATOR_COUNT);
 }
 
-fn get_store(db_path: &TempDir) -> Arc<HotColdDB<E>> {
+fn get_store(db_path: &TempDir) -> Arc<HotColdDB<E, LevelDB<E>, LevelDB<E>>> {
     let spec = E::default_spec();
     let hot_path = db_path.path().join("hot_db");
     let cold_path = db_path.path().join("cold_db");