influxDBOut should validate key names

[danxmoran]

The InfluxDB line protocol spec says that names of tag keys and field keys can't start with _. The influxDBOut node should validate that the data it's pushing doesn't include any keys that violate this restriction.

To help with migration of existing scripts that write _start etc, we could:

1. Add a new boolean property to set "strictness" (or some other name) of the node, defaulting to false
2. Write the new validation logic so that if the node isn't strict, invalid keys are dropped instead of the whole data point being thrown out