Add no-copy block processing cache (#863)

* Add state cache, remove store cache

* Only build the head committee cache

* Fix compile error

* Fix compile error from merge

* Rename state_cache -> checkpoint_cache

* Rename Checkpoint -> Snapshot

* Tidy, add comments

* Tidy up find_head function

* Change some checkpoint -> snapshot

* Add tests

* Expose max_len

* Remove dead code

* Tidy

* Fix bug

beacon_node/beacon_chain/src/beacon_snapshot.rs

@@ -0,0 +1,53 @@
+use serde_derive::Serialize;
+use ssz_derive::{Decode, Encode};
+use types::{BeaconState, EthSpec, Hash256, SignedBeaconBlock};
+
+/// Represents some block and its associated state. Generally, this will be used for tracking the
+/// head, justified head and finalized head.
+#[derive(Clone, Serialize, PartialEq, Debug, Encode, Decode)]
+pub struct BeaconSnapshot<E: EthSpec> {
+    pub beacon_block: SignedBeaconBlock<E>,
+    pub beacon_block_root: Hash256,
+    pub beacon_state: BeaconState<E>,
+    pub beacon_state_root: Hash256,
+}
+
+impl<E: EthSpec> BeaconSnapshot<E> {
+    /// Create a new checkpoint.
+    pub fn new(
+        beacon_block: SignedBeaconBlock<E>,
+        beacon_block_root: Hash256,
+        beacon_state: BeaconState<E>,
+        beacon_state_root: Hash256,
+    ) -> Self {
+        Self {
+            beacon_block,
+            beacon_block_root,
+            beacon_state,
+            beacon_state_root,
+        }
+    }
+
+    /// Update all fields of the checkpoint.
+    pub fn update(
+        &mut self,
+        beacon_block: SignedBeaconBlock<E>,
+        beacon_block_root: Hash256,
+        beacon_state: BeaconState<E>,
+        beacon_state_root: Hash256,
+    ) {
+        self.beacon_block = beacon_block;
+        self.beacon_block_root = beacon_block_root;
+        self.beacon_state = beacon_state;
+        self.beacon_state_root = beacon_state_root;
+    }
+
+    pub fn clone_with_only_committee_caches(&self) -> Self {
+        Self {
+            beacon_block: self.beacon_block.clone(),
+            beacon_block_root: self.beacon_block_root,
+            beacon_state: self.beacon_state.clone_with_only_committee_caches(),
+            beacon_state_root: self.beacon_state_root,
+        }
+    }
+}