Removed dead code
Moved channel hanlding into a cursor

 * Abstracted Drupal 7 cursors to AbstractD7Cursor
 * Added D7ChannelCursor
 * Removed dead code

WARNING: This cause performance regressions we need to fix:

 * Removed static cache handling from Drupal backend
 * More and more queries are done using channel name instead of channel
   database identifiers (at least they are no JOIN)
 * Added a JOIN statement on subscription getter