[meta] Modernize our code-base

We're moving to Drupal 7, so there are some opportunities to leverage more core components, and thus simplify our code-base. Not all of these are likely to happen in Aegir3, but some might have far-ranging effects, so this issue is to collect some of these issues.

#1455216: Implement the base Hostmaster objects as entities
#814296: use Drupal's semaphore/locking system instead of variable_get/set for dispatcher locking
#2049495: Move tasks from node_operations to actions
#2049543: Refactor our queue using the Drupal Queue API
#2049623: Refactor hosting_clients to use Organic Groups