Graphsync fetcher intra-request parallelism

[ZenGround0]

Description

It may be reasonable to request the same selector from multiple peers at once to fulfill a request. In the case one peer is blocking for an entire requestTimeout and other peers can serve the data then this can speed up requests by ~requestTimeout

Acceptance criteria

Design parallel peer fetches per selector within the graphsync fetcher.

Risks + pitfalls

• This may have low benefit if we can set requestTimeout very low.
• This will increase network traffic. Potentially for little speedup gain.

Where to begin

Discussion with @hannahhoward @anorth @frrist

[anorth]

Yes I think we should do something like this in general. I know bitswap is on the other extreme, I think we want something in-between, two or three requests for the same data (which we can terminate once one peer starts responding).

This is part of a bunch of graphsync follow-on work to reach a fast and robust chain sync.