PCT-970: Agent 1.0.8 running but web app says it's not connected

*To be released in 1.0.10.*

Also fixes:
* PCT-934: Agent 1.0.7 not sending data
* PCT-967: Agent becomes unresponsive
* PCT-879: Agent sends keepalive when disconnected

More enhancements:
* Use `time.Ticker` instead of `time.Timer` for keepalive (should have been a `Ticker` all along)
* `percona-agent -status` gets agent's status via API (remote status)
* `SIGUSR1` causes status dump to stdout (percona-agent.log) (local status)
* `SIGHUP` causes agent to reconnect its command websocket
* Add timeout to `SendData()` (for data sender).
* Make data sender limit its runtime to avoid sending forever without a break.
* Add time and Mbps to `data-sender` status.
* Add `data-spooler-count`, `data-spooler-size`, and `data-spooler-oldest` statuses.