deltas: Rework format to allow streaming

deltas: Rework format to allow streaming

There's still some silliness here, but there is now only one opcode
open-splice-and-close, that writes a single chunk from the payload.
This is really all we need for metadata, and small content objects are
also fine with this.

We get some deduplication between content objects by creating a
dictionary for (uid,gid,mode) tuples and xattrs.

This still keeps the operation/payload code in, so we could do
rollsums in a future update easily.