Use a random IP when a host has several IPs.

Using gethostbyname, we will reuse the same (first) IP address for
each request. Here, we choose the IP we use randomly.

This is practical in a case where we have multiple redis read-only
slaves that can't invidually support the full application load, but are
accessible through a single hostname.
Merge remote-tracking branch 'origin/pr/116' into v0.8