This patches fixes a deadlock whenever TransferManager._shutdown is called with cancel==True

Codecov Report

Merging #179 (3da31a7) into develop (e862f6b) will decrease coverage by 0.08%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #179      +/-   ##
===========================================
- Coverage    97.79%   97.71%   -0.09%     
===========================================
  Files           15       15              
  Lines         2408     2404       -4     
===========================================
- Hits          2355     2349       -6     
- Misses          53       55       +2     

Impacted Files	Coverage Δ
s3transfer/futures.py	95.05% <100.00%> (-0.08%)	:arrow_down:
s3transfer/manager.py	97.53% <0.00%> (-0.99%)	:arrow_down:

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@kdaily could you give this one a look? And #144?

Thanks in advance!!