Add whitelist function and some info on stats collected.

Summary: (ZD231) Whitelisting of classifiers can now be done in the same way as blacklisting. Also added some collectd.info() to show the compiled regexps and some basic counters. For the first 5,000 stats, reports every 500. Thereafter reports every 5000.

Test Plan: pytest

Reviewers: carl, neil

Subscribers: dashboard

Differential Revision: http://grizzly.memsql.com/D5635