Retrieval Market Cleanup W/ Node Interfaces

[hannahhoward]

Goals

This PR is a WIP Branch for extracting the retrieval market to a shared component and updating it to spec.

Implementation

This extraction is based off the existing LOTUS code and V0 of the Filecoin retrieval spec: https://filecoin-project.github.io/specs/#systems__filecoin_markets__retrieval_market

The retrieval market extraction plan has a few distinct steps:

1. Update the external interfaces of the retrieval client and provider to match those defined in spec V0
2. Substitute direct dependencies on the LOTUS node in the retrieval client and provider with an adapter that can be implemented by either ndoe
3. Update the internal implementation of the retrieval module to match V0 of the spec (a significant protocol change to support IPLD structures other than UnixFS + CAR file serialization)

This initial extraction does the first two steps.

This means that internal to the module, the old data types and protocols still exists (i.e. we have not done step 3) and there is translation back and forth which makes the implementation slightly awkward.

fixes filecoin-project/go-retrieval-market-project#1
fixes filecoin-project/go-retrieval-market-project#2
fixes filecoin-project/go-retrieval-market-project#3
fixes filecoin-project/go-retrieval-market-project#4

[magik6k]

1 Nit

[hannahhoward]

@magik6k lock added

Possible gotcha we should keep an eye on:
notifySubscribers calls all subscribers, which are provided from external code
if one were to block, the read lock would stay locked
(go-data-transfer has this issue as well, fwiw)

possible solutions include:

• call each callback in go routine (downside lots of go routines)
• copy the subscriber list and release the lock before calling callbacks (has down side of potential extra callback for someone who already unsubscribed)
• call each callback sequentially in a go routine with a timeout

I'd recommend just deferring but worth keeping in mind