Read first and last index first

[vasco-santos]

Freeway returns 200 and starts to stream once it finds the root block. Depending on tooling that generates the CAR file, the root block can be in the first or last CAR file:

• client w3s/nft writes root CID in the last CAR
• DAG Export in first block root CID

An optimisation to improve TTFB would be to read first and last CARs first.