Engine, task, linear_flow unification

In order to move away from the existing flows having their
own implementation of running, start moving the existing
flows to be  patterns that only structure tasks (and impose
constraints about how the group of tasks can run) in useful
ways.

Let the concept of running those patterns be handled by an
engine instead of being handled by the flow itself. This
will allow for varying engines to be able to run flows in
whichever way the engine chooses (as long as the constraints
set up by the flow are observed).

Currently threaded flow and graph flow are broken by this
commit, since they have not been converted to being a
structure of tasks + constraints. The existing engine has
not yet been modified to run those structures either, work
is underway  to remediate this.

Part of: blueprint patterns-and-engines

Followup bugs that must be addressed:
  Bug: 1221448
  Bug: 1221505

Change-Id: I3a8b96179f336d1defe269728ebae0caa3d832d7