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/src/lib.rs

@@ -11,7 +11,8 @@ pub use config::{get_data_dir, get_eth2_testnet_config, get_testnet_dir};
 pub use eth2_config::Eth2Config;
 
 use beacon_chain::events::TeeEventHandler;
-use beacon_chain::migrate::{BackgroundMigrator, HotColdDB};
+use beacon_chain::migrate::BackgroundMigrator;
+use beacon_chain::store::LevelDB;
 use beacon_chain::{
     builder::Witness, eth1_chain::CachingEth1Backend, slot_clock::SystemTimeSlotClock,
 };
@@ -25,12 +26,13 @@ use types::EthSpec;
 /// A type-alias to the tighten the definition of a production-intended `Client`.
 pub type ProductionClient<E> = Client<
     Witness<
-        HotColdDB<E>,
-        BackgroundMigrator<E>,
+        BackgroundMigrator<E, LevelDB<E>, LevelDB<E>>,
         SystemTimeSlotClock,
-        CachingEth1Backend<E, HotColdDB<E>>,
+        CachingEth1Backend<E>,
         E,
         TeeEventHandler<E>,
+        LevelDB<E>,
+        LevelDB<E>,
     >,
 >;