Improve `dogstatsd stats` subcommand memory usage.

[rayz]

The current implementation builds the output table in memory before any writing is done.

We should

1. Add logging to indicate when the status of the command (whether it's waiting for the response or building the output)
2. Limit the output size in pretty mode
3. Offer a new way to output in a programmatic format

[tobz]

We've addressed points 1 and 2 in #889, but still need to add a way to emit a more machine-friendly output format such as JSON or CSV.