Add flush method to wallet

[thunderbiscuit]

This PR cannot be merged before the flush() method is added to bdk (see upcoming PR)

This fixes #114. Took @notmandatory and I half a day to do it all, but we fully tested the new flush method and it did resolve the bug.

Note that at the moment the database() method on the wallet returns an immutable reference, hence why we cannot simply do:

fn flush(
    &self,
) -> Result<(), Error> {
    self.get_wallet().database().borrow().flush()
}

directly in bdk-ffi to trigger the flush.

[notmandatory]

I think this one can be closed in favor of using sqlite db from #116.

[thunderbiscuit]

Agreed.