Create and add MEF commands on background thread

[jcansdale]

What this PR does

• Create MEF commands on the background thread
• Allow menuService.AddCommands to be called from a b/g thread
• Created AsyncMenuPackage that allows IMenuCommandService to be used on b/g tread
• Make PullRequestStatusBarPackage use MEF on b/g thread before using it on Main thread
  • I'm hoping this will help mitigate Extension likely caused X seconds of unresponsiveness #1516

Notes

I attempted to do everything on the background thread, including calling AddCommand on the IMenuCommandService service. There had been reports of this also working on a background thread:
https://gitter.im/Microsoft/extendvs?at=5ab13986458cbde55757fe4e

Unfortunately MenuCommandService.EnsureVerbs includes a call to GetService:
https://gitter.im/Microsoft/extendvs?at=5ab141aaf3f6d24c6898259e

This can only be called on a background thread by a non-AsyncPackage, for exampe:
http://source.roslyn.io/#Microsoft.VisualStudio.LanguageServices/RoslynPackage.cs,106
(IMenuCommandService and IOleCommandTarget) are services that are created for each Package)

IMenuCommandService.AddCommands to be called form a background thread:

// The IDesignerHost and ISelectionService services are requested by MenuCommandService.EnsureVerbs().
// When called from a non-Main thread this would throw despite the fact these services don't exist.
// This override allows IMenuCommandService.AddCommands to be called form a background thread.
protected override object GetService(Type serviceType) =>
    (serviceType == typeof(ISelectionService) ||
    serviceType == typeof(IDesignerHost)) ? null : base.GetService(serviceType);

This allows us to do all menu related initialization on a b/g thread!

[jcansdale]

Needs some (preferably gramatical) xmldoc comments. 😉

[jcansdale]

Made obsolete by #1569.