During a pull operation where the server does not provide sidedata, the client
that requires it should generate them on-the-fly. In the generic case, we need
to wait for the changelog + manifests + filelogs to be added, since we don't
know what the sidedata computers might need: this means rewriting the sidedata
of index entries from within the pull transaction (and no further back) right
after we've added them.

Both Python and C implementations only allow for rewriting the sidedata offset
and length for revs within the transaction where they were created.