Core API: remove Directives parameter, pt1 (#697).

BREAKING CHANGE

The `Directives` parameter is about being completely removed from the Core API.

Already removed from deletion-related Core Service calls and corresponding event listeners.

There is a new static method to get the `Directives` object for the current request (however most plugins will not need it):
{{{
Directives.get()
}}}

Internally the Core maintains the Directives object in a thread-local variable, and removes it once a Jersey request is processed.

See #697.