Refactor agent core startup solo (#89) (#287)

This PR is made of two tightly coupled parts:

* Total rewrite of agent startup logic from module functions -> singleton class. (some other logic was changed in meter to fix wrong forking behavior)
* Provide experimental support for os.fork(), exposed as an option.
* A demo directory to provide easier access to oap/kafka/demoservices (for contributors).

Minor changes:

* Docs: fixed some missed ones over time.
* Fixed a redis bug.
45 files changed