Refresh DNS lookups periodically to account for load balancers that change DNS

[gbrail]

apib today (like ab) does a single DNS lookup at the start of processing, regardless of the number of connections.

This is fine for most servers but for DNS-based load balancing services such as Amazon ELB, performance is not as good as it should be.

Instead, we should experiment with doing a DNS lookup for each concurrent connection, or even with doing a DNS lookup periodically when opening a new connection. This will better help us test the highly variable throughput produced by ELB.