'Fastest' load balancing mode

**Fastest** load balancing mode chooses a destination with the lowest number of connections and the lowest average response time in the given period. Response time can be calculated as time to get the first byte from the destination or the last one.

No files, tests, or entry points are named in the issue, so start by locating the existing load-balancing implementation and its tests. Done means a “Fastest” mode selects the destination with the lowest connection count and average response time, with the response-time measurement defined as either first-byte or last-byte time.

Written by the indexing model from the issue text.