Source context matching optimize

[skomis-mm]

What issue does this PR address?
Optimization made on source context matching (LevelOverrideMap, Match.FromSource()) for netstandard2.1.

Does this PR introduce a breaking change?
No

Please check if the PR fulfills these requirements

• The commit follows our guidelines
• Unit Tests for the changes have been added (for bug fixes / features)

Other information

Linux

BenchmarkDotNet=v0.12.0, OS=debian 11
Intel Core i7-3840QM CPU 2.80GHz (Ivy Bridge), 1 CPU, 4 logical and 2 physical cores
.NET Core SDK=3.1.201
  [Host]     : .NET Core 3.1.3 (CoreCLR 4.700.20.11803, CoreFX 4.700.20.12001), X64 RyuJIT
  Job-CSERBI : .NET Core 2.1.16 (CoreCLR 4.6.28516.03, CoreFX 4.6.28516.10), X64 RyuJIT
  Job-GDNCBZ : .NET Core 3.1.3 (CoreCLR 4.700.20.11803, CoreFX 4.700.20.12001), X64 RyuJIT

Method	Runtime	Mean	Error	StdDev	Ratio
Filter_MatchingFromSource	.NET Core 2.1	5,799.5 ns	51.17 ns	45.36 ns	1.00
Filter_MatchingFromSource	.NET Core 3.1	3,564.5 ns	37.43 ns	35.01 ns	0.61
Logger_ForContext	.NET Core 2.1	2,519.0 ns	15.37 ns	13.62 ns	1.00
Logger_ForContext	.NET Core 3.1	961.8 ns	4.67 ns	4.36 ns	0.38
LevelOverrideMap_GetEffectiveLevel	.NET Core 2.1	1,745.7 ns	11.95 ns	10.60 ns	1.00
LevelOverrideMap_GetEffectiveLevel	.NET Core 3.1	193.8 ns	2.35 ns	2.20 ns	0.11

Windows

BenchmarkDotNet=v0.12.0, OS=Windows 10.0.19041
Intel Core i7-3840QM CPU 2.80GHz (Ivy Bridge), 1 CPU, 8 logical and 4 physical cores
.NET Core SDK=3.1.300
  [Host]     : .NET Core 3.1.4 (CoreCLR 4.700.20.20201, CoreFX 4.700.20.22101), X64 RyuJIT
  Job-KEWXME : .NET Core 2.1.16 (CoreCLR 4.6.28516.03, CoreFX 4.6.28516.10), X64 RyuJIT
  Job-KKEVOV : .NET Core 3.1.4 (CoreCLR 4.700.20.20201, CoreFX 4.700.20.22101), X64 RyuJIT

Method	Runtime	Mean	Error	StdDev	Ratio
Filter_MatchingFromSource	.NET Core 2.1	8,088.7 ns	120.48 ns	112.70 ns	1.00
Filter_MatchingFromSource	.NET Core 3.1	3,369.0 ns	71.49 ns	73.42 ns	0.42
Logger_ForContext	.NET Core 2.1	6,322.1 ns	98.18 ns	91.84 ns	1.00
Logger_ForContext	.NET Core 3.1	785.6 ns	9.05 ns	8.47 ns	0.12
LevelOverrideMap_GetEffectiveLevel	.NET Core 2.1	5,619.1 ns	20.98 ns	18.60 ns	1.00
LevelOverrideMap_GetEffectiveLevel	.NET Core 3.1	185.4 ns	1.63 ns	1.44 ns	0.03

[nblumhardt]

LGTM! 👍