This adds a executor backed job dispatching base class
and has the existing blocking executor use it by running jobs
and dispatching jobs into a sync executor. It also allows
for dispatching jobs into a thread executor, or other executor
via a new '_executor_factory' method that can generate
executors (it can be overriden in the non-blocking executor
to provide your own executors instances).
This does alter the behavior in that now that jobs are dispatched
into an executor we no longer can immediatly know if a job was
dispatched and raised an exception or whether it will raise an
exception in the future, so we now alter the 'local_dispatched'
to just be a boolean that is used to determine if any dispatches
happened (failure or not).
Change-Id: I485770e8f4c85d3833892a453c9fb5168d8f0407
ae9c701f90