Keep sub alive when reading channel

[caleblloyd]

Resolves #499
Resolves #505

Adds GC keepalive calls to ActivityEndingMsgReader in order to prevent sub from being GC'd while the channel reader is being interacted with.

Learned about GCHandle.Alloc from this blog post - Using GC.KeepAlive in async methods

I left RegisterSubAnchor in place for now as I saw it was in-use still in the JetStream project. But if every sub there uses an ActivityEndingMsgReader it should be able to be eliminated.

[mtmk]

Very nice! 😎

[mtmk]

...just checking the tests. they are still passing for me even after removing the GC magic.

[mtmk]

LGTM

This is fixing the issue for SubscribeCore. Tests don't seem to fail (at least for me on my machine) when the GC fix isn't in place but that can be looked into in a follow up PR if you like.

Below are what I had to do to fail the tests when GC fixes weren't in place.

[caleblloyd]

Of course... I had it all working then simplified the tests to DRY it up, and I covered up the original failures. Just refactored them back to use Task.Run which now fail again for me when using the original ActivityEndingMsgReader

[caleblloyd]

Added a new benchmark for Subscribe, a side effect of the GC Handle is that the WaitAsync... TryRead pattern allocates more than ReadAllAsync on the channel reader now. So we will probably want to change our patterns to use more of ReadAllAsync when accessing the sub reader

Method	Mean	Error	StdDev	Gen0	Allocated
SubscribeAsync	1.002 s	0.0076 s	0.0004 s	3000.0000	30.54 MB
CoreWait	1.002 s	0.0016 s	0.0001 s	5000.0000	46.65 MB
CoreRead	1.003 s	0.0112 s	0.0006 s	5000.0000	40.18 MB
CoreReadAll	1.002 s	0.0005 s	0.0000 s	3000.0000	30.62 MB