Fix bug where channel could outlive event loop

[graebm]

Issue:
Crashes in aws-crt-python if a WebSocket outlived an EventLoopGroup.

Investigation:
aws_channel is refcounted. It uses an aws_event_loop, but event loops do not currently support refcounting.

The aws_event_loop_group is refcounted, but aws_channel only currently knows about an individual loop.

So it's possible for the refcount on aws_event_loop_group to go to 0, and all the loops are destroyed, before the channel's refcount goes to zero, and it tries to schedule a task on the loop, and everything explodes because the loop was already cleaned up

Description of changes:
Add new function aws_event_loop_group_acquire_hold_on_group(struct aws_event_loop *); so that channel can refcount the group, even though it only has access to the individual loop

I considered adding a proper refcount to individual the aws_event_loop, but there's a good bit of code that assumes the lifetime of the loops is tied to the lifetime of the group, so I didn't want to break that assumption.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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