Feature: Anomaly counter discovery mode

[jbemmel]

Inspired by a Meta presentation at NANOG89 where they found some 1 in 100000 packet drops counters seen in egress VOQ counters that weren't being monitored:

1. Subscribe on-change to root (/)
2. Say 1000 paths come back. Eliminate 50% of the most frequently changing paths by re-subscribing to sub-paths
3. Repeat step 2 until a shortlist of occasionally changing paths remains -> report in order of change rate frequency (filtering paths that did not change at all)

Step 1 may have to accommodate paths that don't support on-change monitoring