wp-app.php cleanup:

* Put the atom server class in class-wp-atom-server.php
* Remove log_api calls
* Update the query filter
* Replace exit calls with wp_die() for unit test compat
* Add override handler for wp_die()
* Allow plugins to replace the atom class
* Whitespace cleanup

Props kurtpayne. fixes #20042


git-svn-id: https://core.svn.wordpress.org/trunk@20062 1a063a9b-81f0-0310-95a4-ce76da25c4cd