Added caching of event registration state to support unsubscribes across garbage collections

[manodasanW]

This brings back the fix from #876 that was previously reverted.

In addition to the changes then, there is a couple more changes in this PR some of which are fixes for issues that were discovered related to the fix and others were improvements made to see if it resolves some test failures but were kept even if they didn't as they improved the code:

• Updated the EventInvoke delegates to be created by the State class to prevent the delegates from keeping the EventSource alive and thereby the WinRT object
• Made all references to the State object that were cached as weak references. This allows to support the scenario where an object may have events subscribed, but never unsubscribed to not leak when the WinRT object goes away.
• Removed agile reference support from NativeDelegates as it is now provided by IObjectReference.
• Made all the event related ConditionalWeakTables lazy initialized as they were a lot of them on the finalizer queue even though they weren't ever used.

Note that the bulk of the event handler fixes can be seen in WinRT.cs

Fixes #842
Fixes #897
Fixes #858