Add waitable_executor
Validators work (RIPD-703):

This replaces the experimental validators module with foundational
code to implement a new system for tracking validators, validations and
the UNL. The code is turned off by default, in BeastConfig.h

* Remove obsolete public Manager interfaces
* Remove obsolete database methods
* Remove obsolete ChosenList concept
* Remove obsolete code
* Add missing includes
* Tidy up STValidation.h
* Move factory function to Validators::make_Manager
* Add Connection object for tracking STValidations