Add database schema versioning (#1688)

## Issue Addressed

Closes #673

## Proposed Changes

Store a schema version in the database so that future releases can check they're running against a compatible database version. This would also enable automatic migration on breaking database changes, but that's left as future work.

The database config is also stored in the database so that the `slots_per_restore_point` value can be checked for consistency, which closes #673

beacon_node/store/src/metadata.rs

@@ -0,0 +1,29 @@
+use crate::{DBColumn, Error, StoreItem};
+use ssz::{Decode, Encode};
+use types::Hash256;
+
+pub const CURRENT_SCHEMA_VERSION: SchemaVersion = SchemaVersion(1);
+
+// All the keys that get stored under the `BeaconMeta` column.
+//
+// We use `repeat_byte` because it's a const fn.
+pub const SCHEMA_VERSION_KEY: Hash256 = Hash256::repeat_byte(0);
+pub const CONFIG_KEY: Hash256 = Hash256::repeat_byte(1);
+pub const SPLIT_KEY: Hash256 = Hash256::repeat_byte(2);
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+pub struct SchemaVersion(pub u64);
+
+impl StoreItem for SchemaVersion {
+    fn db_column() -> DBColumn {
+        DBColumn::BeaconMeta
+    }
+
+    fn as_store_bytes(&self) -> Vec<u8> {
+        self.0.as_ssz_bytes()
+    }
+
+    fn from_store_bytes(bytes: &[u8]) -> Result<Self, Error> {
+        Ok(SchemaVersion(u64::from_ssz_bytes(bytes)?))
+    }
+}