feat(db): Bootstrap DbExecutor actor

Bootstraps an Actix actor carrying DB connections. This actor will be
used to interact with converse's database.

src/db.rs

@@ -0,0 +1,13 @@
+//! This module implements the database connection actor.
+
+use actix::prelude::*;
+use diesel::prelude::PgConnection;
+use diesel::r2d2::{Pool, ConnectionManager};
+
+/// The DB actor itself. Several of these will be run in parallel by
+/// `SyncArbiter`.
+pub struct DbExecutor(pub Pool<ConnectionManager<PgConnection>>);
+
+impl Actor for DbExecutor {
+    type Context = SyncContext<Self>;
+}