Fix session discard future not being Send

[msrd0]

Fixes #580

[codecov]

Codecov Report

Merging #581 (c64e350) into main (7251955) will increase coverage by 0.33%.
The diff coverage is 95.65%.

@@            Coverage Diff             @@
##             main     #581      +/-   ##
==========================================
+ Coverage   85.95%   86.28%   +0.33%     
==========================================
  Files         112      112              
  Lines        5781     5804      +23     
==========================================
+ Hits         4969     5008      +39     
+ Misses        812      796      -16     

Impacted Files	Coverage Δ
gotham/src/middleware/session/mod.rs	81.76% <95.65%> (+4.63%)	⬆️
gotham/src/middleware/session/backend/memory.rs	96.93% <0.00%> (+6.12%)	⬆️

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