Add support for NSURLCache.

[kgoodier]

The NSURL loading system has subtle and poorly documented behavior
regarding caching. CocoaSPDY was not doing the right thing, thus
there was no support for NSURLCache in the protocol. This patch
adds basic support for both NSURLConnection and NSURLSession based
requests. This is not yet a fully-featured client caching
implementation.

If the request specifies a NSURLRequest cachePolicy of:

NSURLRequestUseProtocolCachePolicy

• NSURL system does not provide a cached response to the protocol
  constructor. It is up to the protocol to load and validate the
  cached response.

NSURLRequestReturnCacheDataElseLoad

• NSURL system will provide the cached response, if available. The
  protocol is expected to validate the response, and load if not
  available or not valid.

NSURLRequestReturnCacheDataDontLoad

• NSURL system will provide the cached response, if available. The
  protocol is expected to validate the response. The protocol will not
  be loaded if no cached response is available, or if one is but is
  invalid, the protocol should not load the request.

In the cases where the protocol has a cached response and it is valid,
it is supposed to call URLProtocol:cachedResponseIsValid. CocoaSPDY
was not doing this. Determining validity of the cached response is the
job of the protocol, and a basic implementation has been provided here.

CocoaSPDY also has to jump through some hoops whenever NSURLSession is
being used, as Apple has not provided a way to get the
NSURLSessionConfiguration and thus we cannot get the right NSURLCache.
Fortunately we have already provided a workaround for this.

SPDYMetadata has been extended to provide the source of the response.

Observationally, we've seen the NSURL loading system fail to insert
items into the NSURLCache when using NSURLSession and iOS 7/8.
NSURLConnection works fine on all these, and NSURLSession works fine on
iOS 9. Best guess is there's a bug in the NSURLProtocol implementation
that fails to insert into the cache.

So, here we are creating a workaround for that specific case. CocoaSPDY
will buffer the data internally, up to a certain size limit based on the
cache size, and will manually insert into the cache when the response is
done. This could potentially be expanded in the future for unclaimed,
finished push responses, but for now those are excluded.

[NSProgrammer]

Looks great! I have a number of comments that should be addressed though. Thanks, Kevin!

[NSProgrammer]

+1

[niw]

Do we need this init override?

[kgoodier]

Nope. :) Leftover.

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