Clean up database abstractions (#1200)

* Remove redundant method

* Pull out a method out of a struct

* More precise db access abstractions

* Move fake trait method out of it

* cargo fmt

* Fix compilation error after refactoring

* Move another fake method out the Store trait

* Get rid of superfluous method

* Fix refactoring bug

* Rename: SimpleStoreItem -> StoreItem

* Get rid of the confusing DiskStore type alias

* Get rid of SimpleDiskStore type alias

* Correction: A method took both self and a ref to Self

beacon_node/src/lib.rs

@@ -10,7 +10,7 @@ pub use client::{Client, ClientBuilder, ClientConfig, ClientGenesis};
 pub use config::{get_data_dir, get_eth2_testnet_config, get_testnet_dir};
 pub use eth2_config::Eth2Config;
 
-use beacon_chain::migrate::{BackgroundMigrator, DiskStore};
+use beacon_chain::migrate::{BackgroundMigrator, HotColdDB};
 use beacon_chain::{
     builder::Witness, eth1_chain::CachingEth1Backend, events::WebSocketSender,
     slot_clock::SystemTimeSlotClock,
@@ -25,10 +25,10 @@ use types::EthSpec;
 /// A type-alias to the tighten the definition of a production-intended `Client`.
 pub type ProductionClient<E> = Client<
     Witness<
-        DiskStore<E>,
+        HotColdDB<E>,
         BackgroundMigrator<E>,
         SystemTimeSlotClock,
-        CachingEth1Backend<E, DiskStore<E>>,
+        CachingEth1Backend<E, HotColdDB<E>>,
         E,
         WebSocketSender<E>,
     >,