httpclientx: fast recovery on error and endpoint index

[bassosimone]

To replace httpapi for test helpers, httpclientx needs two extra functionality:

1. fast recovery in case a previous attempt failed (e.g., we immediately get connection reset by peer and, in such a case, it does not make sense to wait for a schedule interval before starting the next attempt);
2. returning the index of the endpoint that succeeded, which is required to know which test helper we used.

Once we have implemented both, it is safe to move forward and use httpclientx for the test helpers.