Support multiple network interfaces

[swankjesse]

In particular, we want to:

• Allow an OkHttpClient instance to be pinned to a specific network interface (ie. source IP address). For example, pinning to the WiFi interface would cause it to use that interface for both DNS and connections. Nothing would fall back to the cellular radio.
• Be smarter when no network is pinned. If the device's network changes, we may need to clear the connection pool!

[swankjesse]

With the DNS change and pluggable socket factories I think we cover these use cases well enough. Might be worth reconsidering the API for OkHttp 3, but that'll be a separate effort.
#1967