Reduce JSON deserialization calls, add benchmarks

[z4kn4fein]

Describe the purpose of your pull request

Reducing the number of JSON deserialization calls by caching the last deserialized object in memory. Now, the deserialization is called only when the retrieved JSON string differs from the latest cached one.

Performance benchmark:

| Method |       Mean |     Error |    StdDev | Ratio |  Gen 0 | Allocated |
|------- |-----------:|----------:|----------:|------:|-------:|----------:|
|    Old | 109.080 us | 1.7396 us | 1.6272 us |  1.00 | 2.9297 |     25 KB |
|    New |   7.434 us | 0.1480 us | 0.1384 us |  0.07 | 0.9308 |      8 KB |

Related issues (only if applicable)

22

Requirement checklist (only if applicable)

• [ ] I have covered the applied changes with automated tests.
• [x] I have executed the full automated test set against my changes.
• [x] I have validated my changes against all supported platform versions.

[codecov[bot]]

Codecov Report

Merging #24 (fa35873) into master (26b2edd) will increase coverage by 0.02%. The diff coverage is 95.91%.

:exclamation: Current head fa35873 differs from pull request most recent head d6ac458. Consider uploading reports for the commit d6ac458 to get more accurate results

@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
+ Coverage   96.89%   96.91%   +0.02%     
==========================================
  Files          27       27              
  Lines         708      713       +5     
  Branches       71       73       +2     
==========================================
+ Hits          686      691       +5     
  Misses         12       12              
  Partials       10       10              

Impacted Files	Coverage Δ
src/ConfigCatClient/HttpConfigFetcher.cs	96.51% <92.30%> (-0.05%)	:arrow_down:
src/ConfigCatClient/ConfigCatClient.cs	92.42% <100.00%> (-0.06%)	:arrow_down:
src/ConfigCatClient/ConfigDeserializer.cs	100.00% <100.00%> (ø)
src/ConfigCatClient/Evaluate/RolloutEvaluator.cs	97.67% <100.00%> (+0.01%)	:arrow_up:

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[z4kn4fein]

Update:

Replaced .GetHashCode() with MurmurHash3. Far less chance for hashes to collide.

Updated benchmark:

| Method |       Mean |     Error |    StdDev | Ratio |  Gen 0 |  Gen 1 | Allocated |
|------- |-----------:|----------:|----------:|------:|-------:|-------:|----------:|
|    Old | 107.847 us | 0.4968 us | 0.4647 us |  1.00 | 2.9297 | 0.1221 |     25 KB |
|    New |   6.921 us | 0.0347 us | 0.0325 us |  0.06 | 1.3809 | 0.0076 |     11 KB |