influxdb: Refactor common drivers to prepare for tagging

[BrianJKoopman]

Description

This PR refactors the common InfluxDB drivers in order to make it easier to add InfluxDB tagging for #175.

Motivation and Context

This code is fairly old and kind of a mess. It's maybe still kind of messy, but at least a messy that made it easier to add tagging.

How Has This Been Tested?

The first commit (f56539f) here adds a test for the 'json' protocol, which was missing. And 98a825a adds more tests for single 'timestamp' style messages, which were also missing.

Also, throughout development I had the old code still running with an assert statement checking the new code output the same structure. I ran the agent locally using both the 'json' and 'line' protocols.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code refactor (non-breaking changes which change the structure of the code, but have no behavioral changes)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.