Add pct.Cmd to test agent restart.
Remove, don't send empty data files (PCT-764). Fix ws client and how data.Sender and log.Relay use it. Increase and improve testing in log, data, and agent. Add trash to basedir and data.Spooler.Reject (not used yet). Fix pct.WaitStartLock().
Add trash dir to basedir. Fix percona-agent/main.go.
Fix mock data client. Remove unused const from data.Spooler.
Fix mock ws client.
Merge pull request #87 from percona/pct-764-bad-data-files

PCT-764: Bad data file causes agent to stop sending but keep spooling data