Open Radar

 
When switching from HTTP/1.1 to HTTP2 we saw a decrease on our success rate, the culprit seems to be request cancelations. When using an HTTP2 endpoint with iOS9, there seems to be a race condition where a request cancelation ends up closing the socket instead of the individual stream. This has a side effect: while multiplexing requests on specific conditions, other requests might fail throwing:

```
Error Domain=NSURLErrorDomain Code=-1017 "cannot parse response" UserInfo={NSUnderlyingError=0x1556098f0 {Error Domain=kCFErrorDomainCFNetwork Code=-1017 "(null)" UserInfo={NSErrorPeerAddressKey=<CFData 0x1556a3bd0 [0x1a1778b68]>{length = 16, capacity = 16, bytes = 0x100201bb3400a2ec0000000000000000}, _kCFStreamErrorCodeKey=-1, _kCFStreamErrorDomainKey=4}}, NSErrorFailingURLStringKey=https://api-http2.lyft.com/users/65073180/location, NSErrorFailingURLKey=https://api-http2.lyft.com/users/65073180/location, _kCFStreamErrorDomainKey=4, _kCFStreamErrorCodeKey=-1, NSLocalizedDescription=cannot parse response}
```

The code path that gets hit when the necessary conditions are met for the race to happen is:

We call `cancel` on a `NSURLSessionTask` which:
 - ends up calling `-[__NSCFLocalSessionTask _onqueue_cancel_with_error:]`
 - which checks for self->_connKey to be `nil` otherwise:
     - calls `[self invalidateUnpurgeableConnectionsForConnectionCacheKey:]`
     - which ends up closing not only the stream but the entire connection.

in this particular case, when the race occurs _connKey is not `nil` (my guess: because there are multiple streams) and then the connection gets terminated.

You can find:
 - Full CFNetwork diagnostic here: https://gist.github.com/Reflejo/b4ea68e73be3059326ec#file-cfnetwork_http2_break-log
 - The TCP dump for this condition here: https://gist.github.com/Reflejo/2e3f352308d9861955ff
 - The code that reproduces this condition. Note that we are trying it on a throttled(3G) iPhone but the problem exists even running it as a swift script. here: https://gist.github.com/Reflejo/56d280acf78425d79408

Steps to Reproduce:
Run https://gist.github.com/Reflejo/56d280acf78425d79408 on a throttled device pointing to an http2 server

Expected Results:
Cancelations should send a STREAM_RST and not close the stream; not the connection. Also other requests should not be affected by one cancelation

Actual Results:
The connection is terminated and multiplexed requests sometime fail (even the ones that are not being canceled)