add locks around client deals in retrieval market

[vyzo]

Closes #314

[codecov-commenter]

Codecov Report

Merging #315 into master will decrease coverage by 0.41%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #315      +/-   ##
==========================================
- Coverage   63.75%   63.34%   -0.40%     
==========================================
  Files          41       41              
  Lines        2590     2545      -45     
==========================================
- Hits         1651     1612      -39     
+ Misses        818      815       -3     
+ Partials      121      118       -3     

Impacted Files	Coverage Δ
retrievalmarket/impl/client.go	67.34% <100.00%> (+1.85%)	⬆️
retrievalmarket/impl/blockio/traverser.go	68.89% <0.00%> (-3.33%)	⬇️
piecestore/piecestore.go	80.00% <0.00%> (-2.75%)	⬇️

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[vyzo]

ping @hannahhoward -- let's get this merged.

[hannahhoward]

@vyzo sorry I thought I had merged this but I guess maybe I messed up? Anyway, doing now.