feat: store events in SQLite and allow querying

Events are persisted in a local database, and can be queried through
subscriptions.

src/lib.rs

@@ -1,6 +1,7 @@
-pub mod conn;
-pub mod protostream;
-pub mod event;
-pub mod subscription;
 pub mod close;
+pub mod conn;
+pub mod db;
 pub mod error;
+pub mod event;
+pub mod protostream;
+pub mod subscription;