Enable persistent connections

[ob-stripe]

r? @brandur-stripe @remi-stripe
cc @stripe/api-libraries

Instead of creating and closing a new curl handle for each request, create a curl handle once and reuse it for all requests. This should offer a significant performance improvement, since curl will reuse the connection.

Unfortunately, it's difficult to write automated tests for this given the current shape of CurlClient (not really possible to test private static methods). I've ran some manual tests though, and things look pretty good.

This should also be thread-safe. From https://secure.php.net/manual/en/intro.pthreads.php:

Static Members: When a new context is created ( Thread or Worker ), they are generally copied, but resources and objects with internal state are nullified (for safety reasons).

Because CurlClient is instantiated as a singleton via CurlClient::instance(), each thread gets its own client instance (and thus its own curl handle). I've verified that running multiple threads in parallel doesn't cause issues when persistent connections are enabled.

[brandur-stripe]

I don't 100% understand the static thread safety thing — it kind of sounds like we're sort of hoping that a cURL connection is correctly identified as stateful and therefore copied safely. That said, if testing yielded good results I guess it's fine.

LGTM.

[ob-stripe]

it kind of sounds like we're sort of hoping that a cURL connection is correctly identified as stateful and therefore copied safely.

More precisely, it's the static CurlClient instance that's identified as stateful and nullified, so the first time CurlClient::instance() is called in a new thread, a new CurlClient instance will be created.