Safari doesn't like the prepare txn still open when opening the sync txn

Waiting for it to close magically solves the TransactionInactiveError
we were seeing on some incremental sync request when
reading from roomMembers.

Still unsure what this is about, and if we should wait for all read txns
to close or not.

src/matrix/Sync.js

@@ -235,6 +235,7 @@ export class Sync {
         await Promise.all(roomStates.map(async rs => {
             rs.preparation = await rs.room.prepareSync(rs.roomResponse, rs.membership, prepareTxn);
         }));
+        await prepareTxn.complete();
         await Promise.all(roomStates.map(rs => rs.room.afterPrepareSync(rs.preparation)));
     }