Enhancements to GCPerfSim (dotnet#2921)

* Commented out assert, added project and solution for debugging.

* Commented out assert, added project and solution for debugging.

* Remove earlier changes to be even with upstream repo.

* Initial version of simulating request processing in servers.

* Enhancements to GCPerfSim:
- Simulate server "requests" where we hold on to a collection of objects for an intermediate amount of time.
- Enable varying the number of threads between phases
- Enable varying the amount of computation vs. allocation between phases
- Add pre-baked object size distribution to become more realistic in this regard

Details:
- Option "-requestAllocMB" or "-ramb" determines the amount of memory allocated per request in MB.
- Option "-requestLiveMB" or "-rlmb" determines the live memory size per request
- Option "-sizeDistribution" or "sizeDist" determines the object size distribution used - only "1" is supported now
- Slightly relaxed "AboutEquals" method.
- Moved "compute" field from PerThreadArgs to Phase to enable varying between phases.
- Support comments at the start of an input text file.

* Remove unneeded using clause, add comments.

* Add comment

* Adressed code review feedback, added examples text file driving GCPerfSim through a busy phase and an idle phase, undid changes not intended to be checked in.

* Enhanced comment describing -ramb option.

* Commented out assert, added project and solution for debugging.

* Remove earlier changes to be even with upstream repo.