[IVAPP-1189]: Add priority level to jobs
Move priority level to boolean "highPriority"
Clarify priority names and comments
Handle invalid dependent job (missing parent(s)) on load
BUGFIX: Sync updated dependent jobs to zookeeper

Scheduled jobs, when updated, are immediately synced to the
PersistenceStore (usually zookeeper). But dependent jobs were
only updated in memory. This leads to the nasty failure on restart
in the following sequence of events:

  1. create dependent job with parent A
  2. update dependent job with a new parent B
  3. delete old parent A
  4. restart chronos
  5. experience gnashing of teeth and weeping when chronos fails
     to start properly, because it cannot load the dependent job
     (which still requires parent A)

My previous commit
"Handle invalid dependent job (missing parent(s)) on load" prevents
chronos from simply choking on step 5.

This commit fixes the root issue.