Add PieceCid to RetrievalPeer

[ingar]

Summary

Add PieceCID *cid.Cid to RetrievalPeer so that callers can choose peers appropriately.

• exercise local peer resolver in integration test
• use cbor-gen for RetrievalPeer

Resolves #300

[codecov-commenter]

Codecov Report

Merging #312 into master will decrease coverage by 0.42%.
The diff coverage is 70.59%.

@@            Coverage Diff             @@
##           master     #312      +/-   ##
==========================================
- Coverage   63.75%   63.34%   -0.41%     
==========================================
  Files          41       41              
  Lines        2590     2561      -29     
==========================================
- Hits         1651     1622      -29     
- Misses        818      819       +1     
+ Partials      121      120       -1     

Impacted Files	Coverage Δ
retrievalmarket/types.go	44.12% <ø> (ø)
storagemarket/impl/client.go	3.19% <0.00%> (-0.01%)	⬇️
retrievalmarket/discovery/local.go	61.91% <85.72%> (+0.93%)	⬆️
retrievalmarket/impl/blockio/traverser.go	68.89% <0.00%> (-3.33%)	⬇️
filestore/filestore.go	85.72% <0.00%> (-1.12%)	⬇️

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