Option to connect to all IPs behind a hostname in parallel

This introduces a `ConnectionStrategy` interface for creating connections to a `Route`. The default `SimpleConnectionStrategy` implemention preserves the old behavior, passing along hostnames to Socket which will chose one of the underlying IPs. The `ParallelConnectionStrategy` implementation will initiate a TCP handshake to each IP concurrently and return the first connection to complete.

I tested `ParallelConnectionStrategy` with our API endpoint and got the datacenter I'm close to, as expected, and the time it look was comparable to my RTT to that datacenter (5-15ms in most cases).

Somewhat related to issue #506 (Happy Eyeballs).
We have a `MockWebServerRule`! (if you're interested)