Provide a way to disable datadog stats

[ianbamforth]

Having upgraded to the most recent version of synaps, my logs are flooded with messages from collectd:

collectd[1605]: statsd plugin: Unable to parse line: "synapse.watcher.ping.count:1|c|#watcher_name:storereader-raw_ca"

Initial investigation suggests that the statsd implementation introduced recently is formatted specifically for DataDog, so will not work with "standard" collectd. (apologies if I've got this wrong, it's Friday afternoon and I want to go home).

Need a way to turn off the datadogging.