PR 3724 - Library header restructuring

[Alan Carroll <so...@oath.com.INVALID>]

This PR was committed last night and is is major change to how lib/ts, the
C plugin API, and C++ plugin API are arranged. The goal of this is to unify
include paths between plugins and the TS core so that, eventually, plugins
in the TS code base are not privileged compared to third party plugins by
having access to headers/utilities that third party plugins do not. There
is a brief write up on the new structure here -
https://docs.trafficserver.apache.org/en/latest/developer-guide/introduction/header-file-structure.en.html#developer-header-files

Please feel free to ask me any questions, either via email or on IRC.