main reasons behind this:
- make adding/removing tags cheap
- make adding/removing fields cheap
- make parsing cheaper
- make parse -> decorate -> write out bytes metric flow much faster

Refactor serializer to use byte buffer

- fully document aggregator and processor plugins
- improve readme.md

closes #1989

allows for easier addition of a sliding window at a later time.

Also makes `period` be a generic argument for all aggregator plugins.

closes #1726