benchmark: simple benchmark to register shards

[nonsense]

This PR is adding a simple benchmark using the filesystem to measure how long it takes to register a shard.

Initial results on my end are:

➜  dagstore git:(nonsense/benchmark-dagst) go test -bench=. -benchtime=20s -run BenchmarkDagstore
goos: darwin
goarch: amd64
pkg: github.com/filecoin-project/dagstore
cpu: Intel(R) Core(TM) i9-9980HK CPU @ 2.40GHz
BenchmarkDagstore-16                 349          67796798 ns/op
PASS
ok      github.com/filecoin-project/dagstore    30.935s

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The idea here is to build a suite so that we have guidelines on how long it will take miners to re-index their DAGStore, once we do a migration in the upcoming releases.

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67ms / run seems a bit high to me. We should also add more operations, not just RegisterShard