Improve performance of discovery

[OsirisTerje]

It has been found that discovery of test cases are slowed down by the processing of traits.

[navin22]

With PR #406 I am seeing the below performance improvements for 10K empty nUnit tests in a project.

For Discovery: 67.5% perf improvement in console and 55.9% perf improvement in IDE.

Framework/Adapter Version	VS CLI (vstest.console.exe)	VS IDE (VS2017 Latest)
3.8.1/3.8.0	5.24s	8.4s
3.9.0-dev-04564 / 3.9.0-dev-00575	1.7s	3.7s

For Execution: 26.7% perf improvement in console and 22.35% perf improvement in IDE.

Framework/Adapter Version	VS CLI (vstest.console.exe)	VS IDE (VS2017 Latest)
3.8.1/3.8.0	15.65s	21.25s
3.9.0-dev-04564/3.9.0-dev-00575	11.47s	16.5s

The above testing is done in following machine configuration:
OSName : Microsoft Windows 10 Enterprise
OSVersion : 10.0.15063
MachineType : x64-based PC
Processor : Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz
LogicalCores : 8
RAMSize : 15.9589996337891GB