Two important bug fixes to 6.3.0

Fixes #422: This caused mutations executed during the rebase phase of sync to get passed undefined as their argument. The user-visible result depends on what the application does in those mutations, but whatever the result, it should only last until the mutation is confirmed server-side.

Fixes #421: If the very first push or pull failed, exponential backoff would not happen and Replicache would enter a tight loop fetching forever.