receive: ensure callback errors are propagated

[jedevc]

Previously, it was possible for errors returned by an asyncronous callback function to not be propagated, and to instead return "context canceled" from Receive.

This could happen based on this specific sequence of events:

• An asyncronous callback is called from HandleChange, and returns an error, completing the async errorgroup context with an error state.
• Another call to HandleChange completes, and uses the context syncronously, returning context.Canceled.
• This in turn propagates to doubleWalkDiff, which returns the canceled error, instead of calling Wait which would contain the actual error group error.

This behavior is racy, based on exactly how and when the asyncronous callback is called.

The fix to this is relatively straightforward: we just need to make sure that syncronous and asyncronous function calls are kept separate, and don't reuse the same context. This means that a failure in an async callback, doesn't cause later sync ones to fail.

[jedevc]

If you're interested in reproing, you can get this to happen by inserting a fake error return into receiver.asyncDataFunc, and then running go test -v -count=100 -run TestCopySimple ./... - some of the failures will include this error, while others don't propagate it correctly.