Avoid a reverse DNS-lookup for a numeric proxy address

When the proxy was specified as an IP address the RouteSelector would force
a reverse-lookup. This introduced extra latency when the DNS is being slow or
missing. It also then turned the name back into a set of IPs, which could
result in the original IP address not being given priority and
connection attempts being made to IPs besides the one specified by the
original Proxy specifier.

This change distinguishes between whether the proxy InetSocketAddress is
specified with a string, or with an InetAddress. This is referred to as
"unresolved" and "resolved" respectively. If an InetAddress is present
we assume we should use that single address to communicate with the
proxy.

This relies on the InetSocketAddress being created "unresolved": if the
InetSocketAddress is created from a hostname but created "resolved",
the InetAddress is filled in and OkHttp will interpret that as an
instruction to just communicate with just that one IP.
Maybe rename to `getHostString`, and mention that in the comment?