DX Improvement in base class API - exposed to concrete implementations.

[ndobromirov]

Currently there are the 2 monolithic methods:

1. send - does rendering + delivery and post-processing. Proposed solution - factor out the rendering
2. postSend does 3 things: Decides to save or not, handles saving / or not of the rendered output to fields, saves the message if deemed so. proposed solution - move each action in a designated method that could be customized by ancestors (custom plug-ins).

POC can be seen here #36