Messages: add support for tags

Not used right now,
but will be used to specify Kafka topics, for example.
First shot at plog-upshot

It's a bit weird to release this before open sourcing the upshot
ecosystem.

Represents a first effort to work with the plog API "out of tree".
Going fully modular!

A lot is happening here.

The overall idea is to get rid of sinks in favor of filters,
a more powerful abstraction.

We have yet to provide good reporting functionality for those,
but please be patient :)
Introduce filter interface in plog-api

This will be useful to introduce stats reporting for filters.
For the time being more of a NOOP that breaks the API,
and makes us move from anonymous classes to inner classes
in a bunch of places.
build.gradle: add minimal-json

Growing tired of manual JSON :)
We'll use this lightweight and simple JSON implementation for reporting.