Try several IP addresses for one host if one fails

**Issue by [jrudolph](https://github.com/jrudolph)**
_Tuesday Feb 10, 2015 at 15:19 GMT_
_Originally opened as https://github.com/akka/akka/issues/16827_

---

From spray/spray#746:

> The easiest strategy would be to try all the available addresses in a round-robin fashion on failed connection attempts and probably make the strategy configurable.

See https://groups.google.com/d/msg/spray-user/d6XNRVfjGpo/Y3UURzz4mewJ

/cc @sirthias

Start by tracing the HTTP client's connection-failure handling and how it selects a host's available IP addresses. Use the linked spray/spray#746 discussion for the intended round-robin and configurability requirements; done means failed connections try alternate addresses according to a configurable strategy.

Written by the indexing model from the issue text.