Fix checkpoints during uploads not being applied

[simolus3]

This is the Kotlin port of powersync-ja/powersync-js#558, see that PR for a more complete description.

I also had to fix some other quirks in the sync implementation here, notably:

1. The triggerCrudUpload() call when receiving keep-alive messages used to block the entire sync stream from receiving subsequent lines until the upload was completed. This changes that to spawn an async job that is not awaited (the trigger listening on ps_crud will still await this though).
2. There was no way to abort sync iterations (used e.g. for checkpoints with a checksum mismatch). This adds that functionality.

[stevensJourney]

LGTM