Add LRU cache to database (#837)

* Add LRU caches to store

* Improvements to LRU caches

* Take state by value in `Store::put_state`

* Store blocks by value, configurable cache sizes

* Use a StateBatch to efficiently store skip states

* Fix store tests

* Add CloneConfig test, remove unused metrics

* Use Mutexes instead of RwLocks for LRU caches

beacon_node/store/src/state_batch.rs

@@ -0,0 +1,47 @@
+use crate::{Error, HotStateSummary, Store};
+use types::{BeaconState, EthSpec, Hash256};
+
+/// A collection of states to be stored in the database.
+///
+/// Consumes minimal space in memory by not storing states between epoch boundaries.
+#[derive(Debug, Clone, Default)]
+pub struct StateBatch<E: EthSpec> {
+    items: Vec<BatchItem<E>>,
+}
+
+#[derive(Debug, Clone)]
+#[allow(clippy::large_enum_variant)]
+enum BatchItem<E: EthSpec> {
+    Full(Hash256, BeaconState<E>),
+    Summary(Hash256, HotStateSummary),
+}
+
+impl<E: EthSpec> StateBatch<E> {
+    /// Create a new empty batch.
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    /// Stage a `BeaconState` to be stored.
+    pub fn add_state(&mut self, state_root: Hash256, state: &BeaconState<E>) -> Result<(), Error> {
+        let item = if state.slot % E::slots_per_epoch() == 0 {
+            BatchItem::Full(state_root, state.clone())
+        } else {
+            BatchItem::Summary(state_root, HotStateSummary::new(&state_root, state)?)
+        };
+        self.items.push(item);
+        Ok(())
+    }
+
+    /// Write the batch to the database.
+    ///
+    /// May fail to write the full batch if any of the items error (i.e. not atomic!)
+    pub fn commit<S: Store<E>>(self, store: &S) -> Result<(), Error> {
+        self.items.into_iter().try_for_each(|item| match item {
+            BatchItem::Full(state_root, state) => store.put_state(&state_root, state),
+            BatchItem::Summary(state_root, summary) => {
+                store.put_state_summary(&state_root, summary)
+            }
+        })
+    }
+}