Allow interval-based rotation for round robin entries

This patch allows trafficserver to rotate round-robin DNS entries based on interval rather than frequency.

This is beneficial for HTTP keepalive when there are a small amount of client requests scaled over multiple clients.

Thus, for e.g. google which tends to return 4-5 RR/SA entries, the normal behavior would be to open 5 TCP connections to google if five clients requested five objects (or even if a single client requested 5 objects, if strict round robin is enabled).

This patch would help in such situations by making round robin rotation based on an absolute interval, so requests are forwarded to a single TCP connection for each interval. After the interval expires, the DNS RR entry is rotated.