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/rest_api/src/helpers.rs

@@ -8,7 +8,7 @@ use hyper::{Body, Request};
 use itertools::process_results;
 use network::NetworkMessage;
 use ssz::Decode;
-use store::{iter::AncestorIter, Store};
+use store::iter::AncestorIter;
 use types::{
     BeaconState, CommitteeIndex, Epoch, EthSpec, Hash256, RelativeEpoch, SignedBeaconBlock, Slot,
 };