Add beacon.watch (#3362)

> This is currently a WIP and all features are subject to alteration or removal at any time.

## Overview

The successor to #2873.

Contains the backbone of `beacon.watch` including syncing code, the initial API, and several core database tables.

See `watch/README.md` for more information, requirements and usage.

watch/src/database/models.rs

@@ -0,0 +1,67 @@
+use crate::database::{
+    schema::{beacon_blocks, canonical_slots, proposer_info, validators},
+    watch_types::{WatchHash, WatchPK, WatchSlot},
+};
+use diesel::{Insertable, Queryable};
+use serde::{Deserialize, Serialize};
+use std::hash::{Hash, Hasher};
+
+pub type WatchEpoch = i32;
+
+#[derive(Debug, Queryable, Insertable, Serialize, Deserialize)]
+#[diesel(table_name = canonical_slots)]
+pub struct WatchCanonicalSlot {
+    pub slot: WatchSlot,
+    pub root: WatchHash,
+    pub skipped: bool,
+    pub beacon_block: Option<WatchHash>,
+}
+
+#[derive(Debug, Queryable, Insertable, Serialize, Deserialize)]
+#[diesel(table_name = beacon_blocks)]
+pub struct WatchBeaconBlock {
+    pub slot: WatchSlot,
+    pub root: WatchHash,
+    pub parent_root: WatchHash,
+    pub attestation_count: i32,
+    pub transaction_count: Option<i32>,
+    pub withdrawal_count: Option<i32>,
+}
+
+#[derive(Clone, Debug, Queryable, Insertable, Serialize, Deserialize)]
+#[diesel(table_name = validators)]
+pub struct WatchValidator {
+    pub index: i32,
+    pub public_key: WatchPK,
+    pub status: String,
+    pub activation_epoch: Option<WatchEpoch>,
+    pub exit_epoch: Option<WatchEpoch>,
+}
+
+// Implement a minimal version of `Hash` and `Eq` so that we know if a validator status has changed.
+impl Hash for WatchValidator {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.index.hash(state);
+        self.status.hash(state);
+        self.activation_epoch.hash(state);
+        self.exit_epoch.hash(state);
+    }
+}
+
+impl PartialEq for WatchValidator {
+    fn eq(&self, other: &Self) -> bool {
+        self.index == other.index
+            && self.status == other.status
+            && self.activation_epoch == other.activation_epoch
+            && self.exit_epoch == other.exit_epoch
+    }
+}
+impl Eq for WatchValidator {}
+
+#[derive(Clone, Debug, Queryable, Insertable, Serialize, Deserialize)]
+#[diesel(table_name = proposer_info)]
+pub struct WatchProposerInfo {
+    pub slot: WatchSlot,
+    pub proposer_index: i32,
+    pub graffiti: String,
+}