Option to adjust behaviour if InfluxDB is unavailable or having performance issues

[i-ky]

Based on #13.

If InfluxDB is not available or slow to respond...

module is getting backed up trying to write data to influx and not timing out or failing in any way

This will...

cause some silent backlog in zabbix and suddenly no data at all is being inserted into zabbix

As a solution:

• [ ] Warn users about potential issue and insist on the importance of monitoring InfluxDB availability/performance.
• [ ] Test and document influxdb-relay as a workaround.
• [ ] Introduce timeout: configuration option, drop data if it is not possible to insert it into InfluxDB in a timely fashion, assume no/infinite timeout if it's not configured (current behaviour).