Fixed a data race that occasionally causes a crash

[STomperi]

if webSocketTransport.closeConnection() and webSocketTransport.initServer() are called in succession. Fix was verified with Xcode Thread sanitizer

[apollo-cla]

@STomperi: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

[designatednerd]

@STomperi thank you for adding the test! Would you mind checking if making acked use Atomic<Bool> also fixes the crash? I have some concerns about firing this off to a background queue if we don't have to.

[designatednerd]

@STomperi Also, please sign the CLA when you get a chance - I will not be able to merge this without that.

[STomperi]

@designatednerd Sorry for the delay. I tested with Atomic for acked. It fixes the data race for acked, but I still get data race in queue.keys.max() without firing to background queue:

[designatednerd]

Blech, okay. Please sign the CLA, otherwise I can't merge this. Thank you!

[designatednerd]

heads up that i've rebased to fix the merge conflict and am going to force-push to your branch so this becomes mergeable