Create influxdb plugin for mavlink logging

[fnoop]

Supercedes: #500

Instead of trying to read log files in realtime, instead hook straight into a mavlink log stream and get the logging data from there.

Ardupilot: https://mavlink.io/en/messages/ardupilotmega.html#REMOTE_LOG_DATA_BLOCK
PX4?: https://mavlink.io/en/messages/common.html#LOGGING_DATA

Example of python mavproxy module reading data from ardupilot: https://github.com/ArduPilot/MAVProxy/blob/master/MAVProxy/modules/mavproxy_dataflash_logger.py

We could write a collectd or telegraf plugin, or possibly just a straight influxdb plugin:
https://docs.influxdata.com/influxdb/v2.0/write-data/best-practices/optimize-writes/
https://docs.influxdata.com/influxdb/v2.0/reference/api/client-libraries/

The advantage of writing eg. collectd plugin is that it should be agnostic to the backend, ie we can replace influxdb. However writing an influxdb plugin will probably be much faster/performant.

[fnoop]

This should replace mavlogd