Implementing query execution via Thrift API extension socket

[dannykopping]

This PR implements query execution via Thrift API extension socket, with some accompanying refactorings.

I've added this feature since I'm running osqueryd, and have extensions and other custom configurations - which this exporter doesn't currently support. Additionally, with the socket mode, this exporter could be run inside a container.

It also includes some minor fixes to the documentation, and updating of Go to 1.15. I've maintained backwards-compatibility with the configuration file.

[dannykopping]

See https://osquery.readthedocs.io/en/stable/development/osquery-sdk/#extensions for more details on the Thrift API

[andrewchambers]

@dannykopping are you still using your branch? I was thinking about making a fork with your changes.

[dannykopping]

@andrewchambers go for it :+1: