Fix delay showing composer for accounts with large numbers of messages.

* src/client/application/geary-controller.vala
  (GearyController::create_compose_widget_async): Load draft manager and
  email entry completion model asynchronously after the UI has been made
  visible.

* src/client/composer/composer-widget.vala (ComposerWidget): Rename
  set_entry_completions to load_entry_completions, make loading
  async. Make open_draft_manager_async and load_entry_completions public
  so they can be invoked by the controller.

* src/client/composer/contact-list-store.vala (ContactListStore): Load
  contacts asynchronously in smaller batches, so the UI
  remains responsive.