blockjobs: add NULL state

blockjobs: add NULL state

Add a new state that specifically demarcates when we begin to permanently
demolish a job after it has performed all work. This makes the transition
explicit in the STM table and highlights conditions under which a job may
be demolished.

Alongside this state, add a new helper command "block_job_decommission",
which transitions to the NULL state and puts down our implicit reference.
This separates instances in the code for "block_job_unref" which merely
undo a matching "block_job_ref" with instances intended to initiate the
full destruction of the object.

This decommission action also sets a number of fields to make sure that
block internals or external users that are holding a reference to a job
to see when it "finishes" are convinced that the job object is "done."
This is necessary, for instance, to do a block_job_cancel_sync on a
created object which will not make any progress.

Now, all jobs must go through block_job_decommission prior to being
freed, giving us start-to-finish state machine coverage for jobs.

Transitions:
Created   -> Null: Early failure event before the job is started
Concluded -> Null: Standard transition.

Verbs:
None. This should not ever be visible to the monitor.

             +---------+
             |UNDEFINED|
             +--+------+
                |
             +--v----+
   +---------+CREATED+------------------+
   |         +--+----+                  |
   |            |                       |
   |         +--v----+     +------+     |
   +---------+RUNNING<----->PAUSED|     |
   |         +--+-+--+     +------+     |
   |            | |                     |
   |            | +------------------+  |
   |            |                    |  |
   |         +--v--+       +-------+ |  |
   +---------+READY<------->STANDBY| |  |
   |         +--+--+       +-------+ |  |
   |            |                    |  |
+--v-----+   +--v------+             |  |
|ABORTING+--->CONCLUDED<-------------+  |
+--------+   +--+------+                |
                |                       |
             +--v-+                     |
             |NULL<---------------------+
             +----+

Signed-off-by: John Snow <jsnow@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>