Use serde to parse data lines and checkpoints

[simolus3]

This is another item moved out of #70 with the intention of making that PR easier to review.

This replaces SQL queries to traverse JSON structures with a serde deserialization in two places:

1. When validating checksums.
2. When inserting a data batch.

For checkpoints, the implementation has been moved to src/sync/checkpoint.rs, the existing checkpoint.rs delegates to the new function. The new sync client will also log a bit more information on checksum failures (like the checksum from the checkpoint line and the op_checksum and add_checksum in the database) - I've kept that logic here, which means that checkpoint.rs needs to map back into the simple string representation.

Similarly, logic to insert data has moved to src/sync/operations.rs with the existing file just delegating to the new function. This shouldn't change behavior at all (the only change is that the save_data previously accepted oplog entries where data was a direct JSON object - now it needs to be a JSON-serialized string. I can restore the existing behavior here as well, but my understanding is that all client SDKs have been migrated).