Ensure "civicrm_initialized" action only fires once

[christianwach]

Overview

At present, the civicrm_initialized action fires every time civi_wp()->initialize() is called. This should only happen once.

Before

The civicrm_initialized action fires every time civi_wp()->initialize() is called.

After

The civicrm_initialized action fires only when civi_wp()->initialize() actually initialises CiviCRM.

Technical Details

Code that listens for the civicrm_initialized action receives callbacks every time civi_wp()->initialize() is called. This appears to have been an error on my part when refactoring for CiviCRM 4.6. This PR implements the intended behaviour.

[kcristiano]

I have done an r-run on master with this PR applied.

Install is fine, CiviCRM itself is good. The usual set of plugins I use run as expected.

The change is an improvement. If there are any plugins/extensions that are affected we'll flush them out during the RC period.