Support sequential consistency in SQLite implementation (#64)

This is a bit tricky because the `Storage` trait is `Send + Sync`, but
an SQLite connection is neither. Since this is not intended for
large-scale use, the simple solution is just to open a new SQLite
connection for each transaction. More complex alternatives include
thread-local connection pools or a "worker thread" that owns the
connection and communicates with other threads via channels.

This also updates the InMemoryStorage implementation to be a bit more
strict about transactional integrity, which led to a number of test
changes.

server/src/api/get_snapshot.rs

@@ -50,6 +50,7 @@ mod test {
         {
             let mut txn = storage.txn().unwrap();
             txn.new_client(client_id, Uuid::new_v4()).unwrap();
+            txn.commit().unwrap();
         }
 
         let server = WebServer::new(Default::default(), None, storage);
@@ -86,6 +87,7 @@ mod test {
                 snapshot_data.clone(),
             )
             .unwrap();
+            txn.commit().unwrap();
         }
 
         let server = WebServer::new(Default::default(), None, storage);