Allow manual checkpointing

[northeastprince]

It'd be nice if there was a way to manually checkpoint the WAL file if you need everything in one file at a certain moment.

[flavorjones]

@northeastprince Thanks for asking about this. Can you share a bit more information about your use case? Adding a wrapper for sqlite3_wal_checkpoint_v2() wouldn't be hard, but I do want to know what it's being used before committing to adding it to the gem.

[flavorjones]

You may be able to use the pragma for this: https://www.sqlite.org/pragma.html#pragma_wal_checkpoint