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
Simplify RipplePublicKey:

This implements the bare minimum necessary to store a 33 byte public
key and use it in ordered containers. It is an efficient and well
defined alternative to RippleAddress when the caller only needs
a node public key.
Remove unused and obsolete Ripple identifiers and tidy up:

These identifiers were part of a failed set of classes to replace
the functionality combined into RippleAddress. They are not used
and therefore can be removed.

* Remove RippleAccountPrivateKey
* Remove RippleAccountPublicKey
* Remove RippleAccountID
* Remove RipplePrivateKey
* Remove RipplePublicKeyHash
* Remove RippleLedgerHash
* Remove unused withCheck argument
* Remove CryptoIdentifier
* Remove IdentifierStorage
* Remove IdentifierType
* Add missing include