Modifies WAServerManager to support subclasses to be defined as default

[eMaringolo]

VAST, and probably other dialects, have a subclass of WAServerManager that is used instead of it.

Since WAServerManager[default] is a class instance variable, then sending the message default to WAServerManager will always answer an instance of itself. Which is more troubling if you access it by means of WAAdmin defaultServerManager.

This PR modifies the default class instance variable to be a class variable (named Default), and provides a beDefault method that can be used on subclasses to be the "singleton" default.

[marschall]

Out of curiosity, why do you need to subclass WAServerManager? Is there something we can do so that you don't have to do this?

[eMaringolo]

The WASstServerManager in VAST provides logging of the start/stop events and also a callback interface (think of an "Announcer") to notify these events to the visual control panel can reflect the state without unnecessary polling.

[jbrichau]

Hi Esteban. Thanks for this and sorry for having skipped this PR several weeks ago.
I just wanted to ask a name change for the method initializeDefaultValueHolder to reflect it does not reinitialize. I suggest ensureDefaultValueHolder.

I'm all pro using class vars rather than class instvars for this. I'm not sure if this was a mistake or deliberate choice. It seems strange for a 'default' singleton implemention though, so I suspect the former.