Use ConcurrentDictionary for options properties.

[JimBobSquarePants]

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Description

Fix #1230 #1224

The two dictionaries used in Configuration and DefaultImageProcessorContext were not thread safe and would throw when accessed in a parallel context.

I've replaced them with ConcurrentDictionary instances which fixes the issue but I wonder whether we should be exposing the property as IDictionary<object, object> over the concrete type since we require thread safety.

[codecov]

Codecov Report

Merging #1234 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1234      +/-   ##
==========================================
- Coverage   82.54%   82.54%   -0.01%     
==========================================
  Files         697      697              
  Lines       30512    30511       -1     
  Branches     3469     3469              
==========================================
- Hits        25187    25186       -1     
  Misses       4605     4605              
  Partials      720      720              

Flag	Coverage Δ
#unittests	82.54% <100.00%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
src/ImageSharp/Configuration.cs	100.00% <100.00%> (ø)
src/ImageSharp/GraphicOptionsDefaultsExtensions.cs	100.00% <100.00%> (ø)
...Processing/DefaultImageProcessorContext{TPixel}.cs	100.00% <100.00%> (ø)

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[antonfirsov]

LGTM.