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/network/src/router/processor.rs

@@ -13,7 +13,6 @@ use itertools::process_results;
 use slog::{debug, error, o, trace, warn};
 use ssz::Encode;
 use std::sync::Arc;
-use store::Store;
 use tokio::sync::mpsc;
 use types::{
     Attestation, ChainSpec, Epoch, EthSpec, Hash256, SignedAggregateAndProof, SignedBeaconBlock,