Allow callers to pass in a custom host resolver implementation.

An HTTP client interacts with the network in two main ways: DNS
lookups and connections to HTTP servers. OkHttp already abstracts
the latter by allowing callers to pass in custom SocketFactory
objects, but does not yet abstract the former.

This change takes the existing internal Dns interface, which is
currently used for testing, and turns it into a publicly
accessible HostResolver interface. This allows callers to
completely abstract all network interaction points.

Examples of what this can be used for:

1. Use alternative DNS implementations with different
   performance / caching / ordering / parallelization / ...
   characteristics than standard InetAddress.getAllByName.
2. Resolve hosts using different DNS servers than the system
   resolvers, or even non-DNS protocols (e.g., MDNS or even
   NetBIOS/WINS) that are not supported by the system resolver.
3. Do DNS lookups on specific networks, similarly to what
   android_getaddrinfofornet does.

Change-Id: I6e488acd938067e4c078c6ffe4d5eddb5f3951de
nit: `**` comment for javadoc