Extensions - Define the upgrader base class

Extensions - Define the upgrader base class

This class is based on refactoring the civix template.

Major sections have been split out into traits.  This should make it more
readable (and potentially make it easier to remix).

This base-class should generally provide an equivalent DX for subclass authors:

* Variables have the same names.
* Most method signatures are identical (e.g.  `executeFoo()`)
* Some methods have been redeclared in equivalent form (`addTask()` - using splat instead `func_get_args()`).
* Two internal functions have slightly diff signatures (`enqueuePendingRevisions()`, `_queueAdapter()``).