Use of events and/or polling in the SKA Telescope Control System

Purpose of this architectural decision is to define guidelines for use of events and polling in the SKA Control System.  

Background: Work on re-factoring of the TMC Central Node re-surfaced long standing discussion regarding the use of events versus polling. This discussion is mostly related to TANGO code, but the discussion and guidelines should not be limited to TANGO based code. We should define both the general guidelines and TANGO-specific guidelines.

For more information see: 

 https://confluence.skatelescope.org/display/SWSI/ADR-52++Use+of+events+and+polling+in+the+Telescope+Control+System