Add power-of-two picker

[jchadwick-buf]

Hmm, this almost seems too simple.

[linear]

TCN-1915 Add power-of-two picker

This picker randomly selects two connections and then returns the one with fewer active requests.

TCN-1866 Add default picking algorithms

Out-of-the-box, we want to provide round-robin, least-loaded, random, and power of two (pick two at random and use the least loaded between them).

[jhump]

Looks great!