Loosen up serialization to be about printing only.

Loosen up serialization to be about printing only.

This allows us to decompress, match and check access() for docids
that arrive out-of-order, instead of serializing their processing.
Especially the access() checking is good to have overlapping other
I/O if possible, even though it's synchronous.

If we ever get async access(), we'll need to rework Serializer
again, probably.