add locks protecting retrieval market maps

[vyzo]

Closes #301
But see also #310, stuff never gets removed from these maps.

[codecov-commenter]

Codecov Report

Merging #311 into master will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #311      +/-   ##
==========================================
+ Coverage   63.52%   63.63%   +0.12%     
==========================================
  Files          41       41              
  Lines        2582     2590       +8     
==========================================
+ Hits         1640     1648       +8     
  Misses        820      820              
  Partials      122      122              

Impacted Files	Coverage Δ
retrievalmarket/impl/provider.go	70.32% <100.00%> (+1.30%)	⬆️

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[hannahhoward]

Hi @vyzo!

Happy to merge this but FYI, the reason I didn't address 301 earlier is that we're about to merge some seperate big changes to retrieval which should obviate this issue. SO I would hold on 310

[vyzo]

fair enough, let's merge it to avoid panics in our tests and we will retest with the big bawl of changes!