Move date cache to one thread and box date string to avoid threading issues

[adam-fowler]

Not sure if this is thread safe

[codecov]

Codecov Report

Merging #102 (c35bee4) into main (4e338e2) will decrease coverage by 0.16%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #102      +/-   ##
==========================================
- Coverage   82.91%   82.74%   -0.17%     
==========================================
  Files          58       58              
  Lines        2687     2684       -3     
==========================================
- Hits         2228     2221       -7     
- Misses        459      463       +4     

Impacted Files	Coverage Δ
...rces/Hummingbird/Extensions/EventLoopStorage.swift	93.33% <ø> (ø)
Sources/Hummingbird/HTTP/MediaType.swift	93.97% <ø> (ø)
...urces/Hummingbird/Middleware/MiddlewareGroup.swift	100.00% <ø> (ø)
Sources/Hummingbird/Router/EndpointResponder.swift	81.81% <ø> (ø)
Sources/Hummingbird/Router/TrieRouter.swift	94.82% <ø> (ø)
Sources/Hummingbird/Server/Request.swift	96.36% <ø> (ø)
Sources/Hummingbird/Server/ResponsePatch.swift	85.71% <ø> (ø)
...rces/Hummingbird/Storage/MemoryPersistDriver.swift	77.77% <ø> (ø)
...on/Codable/URLEncodedForm/URLEncodedFormNode.swift	97.27% <ø> (ø)
Sources/HummingbirdFoundation/Files/FileIO.swift	90.72% <ø> (ø)
... and 4 more

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[adam-fowler]

Looks like it isn't. Closing