1. StartStopActivityComputer.cs : support activity grouping node; fix the
activity path string issue (//)
2. ThreadTimeComputer.cs: support custom trace events source.
3. SymbolReader.cs: support conditional build for Symbol_Lookup_At_Collection_Time. In Service Profiler, the agent doesn't need symbol lookup but the server does.
4. TraceEvent.csproj: the PERFVIEW defintion was missing in debug build.
5. TraceEventStacks.cs: support custom module file filter on
lookupwarmsymbols.
6. ZippedETL.cs: support CompressionLevel in WriteArchive function.
7. Support symbolsource.org and nuget symbol/source server.
* Nuget symbol/source server use https://nuget.smbsrc.net
* Nuget symbol/source server use upper case of pdb index guid
* The SRCSRV in Nuget symbol pdb may not have HTTP_ALIAS variable.