Handle re-sent deal proposals

[ingar]

Problem

There is an edge case in the deal flow where a client sends the deal proposal and shuts down for some reason. When the client restarts, it resends the deal proposal, but the provider has already started tracking the deal, and rejects the proposal.

Solution

There is no problem with re-sending the deal response, and not changing anything in the provider state. This will allow the deal to continue on the client's state machine.

Resolves part of filecoin-project/lotus#3966

[codecov-commenter]

Codecov Report

Merging #423 into master will increase coverage by 0.31%.
The diff coverage is 63.64%.

@@            Coverage Diff             @@
##           master     #423      +/-   ##
==========================================
+ Coverage   66.02%   66.33%   +0.31%     
==========================================
  Files          48       48              
  Lines        3178     3189      +11     
==========================================
+ Hits         2098     2115      +17     
+ Misses        846      830      -16     
- Partials      234      244      +10     

Impacted Files	Coverage Δ
storagemarket/impl/provider.go	28.90% <63.64%> (+5.49%)	⬆️

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[hannahhoward]

LGTM