Switched to 3.9.43.0 AssemblyVersion (consistent with other libraries)

All other assemblies in ServiceStack are built with an AssemblyVersion
matching the overall "product" version, currently 3.9.43.
Switched to GlobalAssemblyInfo approach for AssemblyVersion.

All projects in the ServiceStack.sln solution now pull their
AssemblyVersion attribute value from a single GlobalAssemblyInfo.cs file
at the src root, allowing this to be updated centrally. I did not adjust
the Android, SL, or WP projects.