Add support for 32GiB sectors to devnet

We should add support for 32GiB sectors to the devnet in a way that allows developers to iterate on retrieval codebase without too much overhead.

Specifically this means:
- [ ] add support for 32GiB sectors
- [ ] create a genesis block with 2 presealed 32GiB sectors -- include a marker file that detects if this step has been run on developers' machine
- [ ] create a predefined set of car files that a developer can easily store and retrieve in a fast feedback loop, similar to the `autodeal` tool -- https://github.com/nonsense/autodeal/blob/master/spx-notes.json
- [ ] add a marker file around the `lotus fetch-params 34359738368` in order to speed up devnet startup