Use sub encoder

[HDegroote]

Marked as draft because I'm not sure exactly which parts of the code related to batches were work-arounds and which were on purpose. Note in particular that I am now passing the blobs through as a parameter instead of loading it again from this.db in a batch, which enables me to use the _db arg to pass the batch and to get rid of the _files arg (I couldn't figure out why a batch needed to reload the blobs, but there might have been a reason).

There's also a TODO marked in the PR to verify something in the close logic, where we recently made a fix with comment 'workaround to flush the batches for now.' => I think that's no longer relevant, but sanity checking that before fully removing