Support client-side load balancing with the connection pool

How it could work:

• Allow customization of DNS resolution (see also #92) which may return multiple addresses per host
• Allow overriding of target addresses when creating a connection
• In the pool, use different target addresses per slot
• Maybe a custom scheduling is also needed so that sequential requests don't all go the same then-idle connection

One way that users could hack in the first parts without changes to akka-http would be to use a custom ClientTransport returning a Flow that would transparently connect to different addresses each time it is materialized.

See also #1226