Restore speedy non-selser serialization
- Fixes the large perf regression Flow was/is seeing.
- In I104982e061abf20c63a6342caa5600a090c9ced7, we made sure that non-selser didn't need the original source.
- Now, if an oldid isn't supplied, we don't fetch the source and fall back to non-selser. This used to be case before the refactor in I27176bd4b62562a7aead3d40679d31ad0b8f3ab8 (see the review there where this was all discussed).