BibSched: fix spawn_task with monotasks
BibSched: fix spawn_task with monotasks

* Fixes the new spawn_task handling of monotasks by properly
  calling wait() when the task need to block bibsched
  until the end of the process.

inveniosoftware/invenio,tiborsimko/invenio,tiborsimko/invenio,inveniosoftware/invenio