Several things in this commit:

  * Prior to this commit, libplab depended on db state to create slivers
    and slices.  Now it can be done using the regular command line tools
    without the metadata in the db.  This makes development and debugging
    much easier and allows us to use the command-line tools even if state has
    been cleared out of the db (i.e., for sliver garbage collection).
  * Add support for sliver start/stop/restart via the v4 NM.
  * Some support for sliver garbage collection.
  * Various other improvements and cleanup.