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/add_snapshot.rs

@@ -73,6 +73,7 @@ mod test {
             let mut txn = storage.txn().unwrap();
             txn.new_client(client_id, version_id).unwrap();
             txn.add_version(client_id, version_id, NIL_VERSION_ID, vec![])?;
+            txn.commit()?;
         }
 
         let server = WebServer::new(Default::default(), None, storage);
@@ -115,6 +116,7 @@ mod test {
         {
             let mut txn = storage.txn().unwrap();
             txn.new_client(client_id, NIL_VERSION_ID).unwrap();
+            txn.commit().unwrap();
         }
 
         let server = WebServer::new(Default::default(), None, storage);