Rendering deltas with rates

Given two snapshots, and the interval between them, we can compute the rate that derive counters are increasing by.  This patch adds new functions for rendering two snapshots that includes rates for any derives.  (The rate portion is left blank for gauges.)

The periodic interface has also been updated to keep track of the previous snapshot, along with the current one, to make it easy for background threads to calculate rates.
Another option is to fold this into `bws_render_rate` and print this empty string if `rate_per_sec == 0`.