discoverInterfaceName should resolve hostnames before trying to find network interface stats using IP addresses

The code to find a matching network interface to pull stats for assumes the configured interaface is an IP address, but it's just as valid (if not more) to specify a hostname instead.

When the interface is a hostname, discoverInterfaceName() fails to find a matching address from the list of system network interfaces, because it's still trying to match against a hostname instead of an address.

It would be handy if Sync Gateway performed a lookup to fetch IP addresses for a hostname if an IP address was not specified in the first place, before falling through to the existing code. This ensures machines that are able to resolve an address can accurately map it back to a given network interface.