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/compat.rs

@@ -0,0 +1,49 @@
+//! Implementations of PostgreSQL compatibility traits.
+use crate::database::watch_types::{WatchHash, WatchPK, WatchSlot};
+use diesel::deserialize::{self, FromSql};
+use diesel::pg::{Pg, PgValue};
+use diesel::serialize::{self, Output, ToSql};
+use diesel::sql_types::{Binary, Integer};
+
+use std::convert::TryFrom;
+
+macro_rules! impl_to_from_sql_int {
+    ($type:ty) => {
+        impl ToSql<Integer, Pg> for $type
+        where
+            i32: ToSql<Integer, Pg>,
+        {
+            fn to_sql<'a>(&'a self, out: &mut Output<'a, '_, Pg>) -> serialize::Result {
+                let v = i32::try_from(self.as_u64()).map_err(|e| Box::new(e))?;
+                <i32 as ToSql<Integer, Pg>>::to_sql(&v, &mut out.reborrow())
+            }
+        }
+
+        impl FromSql<Integer, Pg> for $type {
+            fn from_sql(bytes: PgValue<'_>) -> deserialize::Result<Self> {
+                Ok(Self::new(i32::from_sql(bytes)? as u64))
+            }
+        }
+    };
+}
+
+macro_rules! impl_to_from_sql_binary {
+    ($type:ty) => {
+        impl ToSql<Binary, Pg> for $type {
+            fn to_sql<'a>(&'a self, out: &mut Output<'a, '_, Pg>) -> serialize::Result {
+                let b = self.as_bytes();
+                <&[u8] as ToSql<Binary, Pg>>::to_sql(&b, &mut out.reborrow())
+            }
+        }
+
+        impl FromSql<Binary, Pg> for $type {
+            fn from_sql(bytes: PgValue<'_>) -> deserialize::Result<Self> {
+                Self::from_bytes(bytes.as_bytes()).map_err(|e| e.to_string().into())
+            }
+        }
+    };
+}
+
+impl_to_from_sql_int!(WatchSlot);
+impl_to_from_sql_binary!(WatchHash);
+impl_to_from_sql_binary!(WatchPK);