add deal creation time to deals

[whyrusleeping]

should help us close filecoin-project/lotus#2646

[hannahhoward]

LGTM not sure what you want to do about tests and time.Now -- I can take this on if you want.

[hannahhoward]

Sorry LGTM after tests and mod-tidy-check are passing

[codecov-commenter]

Codecov Report

Merging #373 into master will decrease coverage by 0.62%.
The diff coverage is 2.92%.

@@            Coverage Diff             @@
##           master     #373      +/-   ##
==========================================
- Coverage   61.78%   61.16%   -0.61%     
==========================================
  Files          41       43       +2     
  Lines        2857     2814      -43     
==========================================
- Hits         1765     1721      -44     
- Misses        941      945       +4     
+ Partials      151      148       -3     

Impacted Files	Coverage Δ
storagemarket/impl/client.go	2.90% <0.00%> (+0.18%)	⬆️
storagemarket/impl/client_environments.go	0.00% <0.00%> (ø)
storagemarket/impl/provider.go	3.01% <0.00%> (+0.48%)	⬆️
storagemarket/impl/provider_environments.go	0.00% <0.00%> (ø)
storagemarket/types.go	0.00% <ø> (ø)
...mpl/requestvalidation/unified_request_validator.go	75.00% <33.34%> (ø)
storagemarket/impl/requestvalidation/common.go	87.10% <100.00%> (-0.40%)	⬇️
piecestore/piecestore.go	38.47% <0.00%> (-26.40%)	⬇️

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