Simplify Akka HTTP client configuration

**Issue by [ktoso](https://github.com/ktoso)**
_Tuesday Mar 15, 2016 at 17:24 GMT_
_Originally opened as https://github.com/akka/akka/issues/20042_

---

Currently it's somewhat confusing to everyone – too much nesting and places where one can set the idle-timeout.

Also, the `akka.http.host-connection-pool.idle-timeout` should be renamed, as it's not "that" idle-timeout, but another one (about shutting down the entire pool when no connections active).

Start by reviewing the Akka HTTP client configuration, especially the nested idle-timeout settings and akka.http.host-connection-pool.idle-timeout. Clarify which settings should remain and how the pool-shutdown timeout should be named; done means the configuration is less nested and the two idle-timeout meanings are distinct.

Written by the indexing model from the issue text.