Fix for SR-2913 - HTTP headers not picked up

[pushkarnk]

In startNewTransfer(with:) though we first set currentRequest property to the incoming request here, the update happens asynchronously on the taskAttributesIsolation queue. Subsequently, in curlHeaders(for: URLRequest) we use the currentRequest property assuming it has been set to get the HTTP headers.

If we end up picking up the HTTP headers before async block runs, we can get into problems. For new requests, the currentRequest property could still be nil, meaning the expected headers will not passed to libcurl. For redirections, we may pick up headers from the previous request.

I think it must be acceptable to use the incoming URLRequest parameter instead of the property. After all, the currentRequest property is expected to be set to the incoming URLRequest.

[pushkarnk]

@swift-ci please test

[pushkarnk]

Another problem reported currentRequest equal to nil here.

The taskAttributesIsolation queue which is used to make the public attributes thread safe should be a concurrent queue and writes should use barriers. Comments on line 74 make this intention clear but this wasn't implemented.

I've made suitable changes to the PR. Two versions of the problem documented in the SR are fixed with this.

[pushkarnk]

@swift-ci please test

[parkera]

LGTM

[pushkarnk]

@swift-ci please test

[pushkarnk]

Looks like a compiler crash. Trying one more time.
@swift-ci please test

[pushkarnk]

@swift-ci please test