ipld: race in Retriever

[Wondertan]

The race is found in #702 branch, but locally.

=== RUN   TestShareAvailable_OneFullNode
2022-06-11T00:07:38.964+0200    ERROR   engine  decision/engine.go:691  wantlist index doesn't match peer's wantlist    {"peer": "1WRsHVXq899cA7"}
2022-06-11T00:07:38.989+0200    ERROR   engine  decision/engine.go:691  wantlist index doesn't match peer's wantlist    {"peer": "1WRsHVXbUUKN1C"}
2022-06-11T00:07:39.207+0200    ERROR   engine  decision/engine.go:691  wantlist index doesn't match peer's wantlist    {"peer": "1WRsHVXbUUKN1C"}
2022-06-11T00:07:39.503+0200    ERROR   engine  decision/engine.go:691  wantlist index doesn't match peer's wantlist    {"peer": "1WRsHVXecAgMfs"}
==================
WARNING: DATA RACE
Read at 0x00c0005ecc30 by goroutine 82302:
  github.com/celestiaorg/celestia-node/ipld.(*retrievalSession).doRequest.func1.1()
      /home/Wondertan/src/go/celestia-node/ipld/retriever.go:241 +0xe4
  github.com/celestiaorg/celestia-node/ipld.GetShares.func1()
      /home/Wondertan/src/go/celestia-node/ipld/get_shares.go:95 +0x242
  github.com/gammazero/workerpool.startWorker()
      /home/Wondertan/go/pkg/mod/github.com/gammazero/workerpool@v1.1.2/workerpool.go:233 +0x35
  github.com/gammazero/workerpool.(*WorkerPool).dispatch.func2()
      /home/Wondertan/go/pkg/mod/github.com/gammazero/workerpool@v1.1.2/workerpool.go:195 +0x47

Previous write at 0x00c0005ecc30 by goroutine 29115:
  github.com/celestiaorg/celestia-node/ipld.(*retrievalSession).doRequest.func1.1()
      /home/Wondertan/src/go/celestia-node/ipld/retriever.go:242 +0x144
  github.com/celestiaorg/celestia-node/ipld.GetShares.func1()
      /home/Wondertan/src/go/celestia-node/ipld/get_shares.go:95 +0x242
  github.com/gammazero/workerpool.worker()
      /home/Wondertan/go/pkg/mod/github.com/gammazero/workerpool@v1.1.2/workerpool.go:243 +0x34
  github.com/gammazero/workerpool.startWorker.func1()
      /home/Wondertan/go/pkg/mod/github.com/gammazero/workerpool@v1.1.2/workerpool.go:234 +0x39

Goroutine 82302 (running) created at:
  github.com/gammazero/workerpool.(*WorkerPool).dispatch()
      /home/Wondertan/go/pkg/mod/github.com/gammazero/workerpool@v1.1.2/workerpool.go:195 +0x41b
  github.com/gammazero/workerpool.New.func1()
      /home/Wondertan/go/pkg/mod/github.com/gammazero/workerpool@v1.1.2/workerpool.go:37 +0x39

Goroutine 29115 (running) created at:
  github.com/gammazero/workerpool.startWorker()
      /home/Wondertan/go/pkg/mod/github.com/gammazero/workerpool@v1.1.2/workerpool.go:234 +0xa4
  github.com/gammazero/workerpool.(*WorkerPool).dispatch.func2()
      /home/Wondertan/go/pkg/mod/github.com/gammazero/workerpool@v1.1.2/workerpool.go:195 +0x47
==================

[Wondertan]

Note that I already have a fix for it and will post after adding more comments in the code with some explanations