Improve speed of GetRegionFromEndpoint with caching

[ngbrown]

I was pre-signing around 3,000 URLs in a single document and noticed a significant delay. I tracked it down to the GetRegionFromEndpoint.

I added a benchmarking project and eventually settled on caching as the fastest solution. This is still an improvement on #394 with more than double the speed of that significant improvement:

BenchmarkDotNet=v0.12.1, OS=Windows 10.0.18363.778 (1909/November2018Update/19H2)
Intel Core i7-6700HQ CPU 2.60GHz (Skylake), 1 CPU, 8 logical and 4 physical cores
.NET Core SDK=3.1.201
  [Host]     : .NET Core 2.1.17 (CoreCLR 4.6.28619.01, CoreFX 4.6.28619.01), X64 RyuJIT
  DefaultJob : .NET Core 2.1.17 (CoreCLR 4.6.28619.01, CoreFX 4.6.28619.01), X64 RyuJIT

Method	Mean	Error	StdDev
OriginalAws	1,068,177.22 ns	16,354.928 ns	14,498.218 ns
OriginalNonAws	150,811.90 ns	2,248.922 ns	2,103.643 ns
OriginalNonAwsPort	3,838,383.62 ns	36,755.305 ns	32,582.620 ns
PreCompiledAws	1,934.85 ns	19.688 ns	19.336 ns
PreCompiledNonAws	313.57 ns	4.027 ns	3.767 ns
PreCompiledNonAwsPort	200.38 ns	1.670 ns	1.481 ns
CurrentAws	82.25 ns	1.720 ns	2.048 ns
CurrentNonAws	60.71 ns	0.465 ns	0.412 ns
CurrentNonAwsPort	74.68 ns	0.585 ns	0.519 ns

[ngbrown]

@geluk, as the author of #394, you might be interested in this improvement.

[geluk]

That's a nice further gain in performance indeed. I'll add on to this that if you provide the region name when instantiating the MinioClient, the GetRegionFromEndpoint method is not called at all, which could be helpful if you're really looking to get as much performance out of it as possible, as it avoids any risk of lock contention in the concurrent dictionary.

[ngbrown]

@geluk. Good point. The client didn't really encourage the passing that in, especially with how the minio server doesn't display if/what the default region is.

ConcurrentDictionary does have a have a lock-free read path.

[Praveenrajmani]

LGTM