Transitioning Cache Digests from "experimental" to fundamental.

Transitioning Cache Digests from "experimental" to fundamental.
Removed a lot of stats collection, espeically large histograms.
Made peerDigestInit() called from an event, instead of being
called directly -- this avoids some recursion.
The selection algorithm is still kind of hard coded as follows:
	cache digests
	netdb (if CD's miss)
	ICP (if netdb has no RTT data)