Get rid of TaskDetails in favor of TaskStub

Change I0b406e4662cfd3cb496b71be77ad10a90c178baa introduces a new
class TaskStub. This class is designed to be a subset of what a
task is in the domain layer: it doesn't contain input, message and
result. TaskStub should be used when listing tasks in order to avoid
extra-overhead of transporting these text fields.
As a result, the TaskDetails introduced by change
I0b406e4662cfd3cb496b71be77ad10a90c178baa doesn't need to exist.

With this patch, the domain layer contains Task and TaskStub instead
of Task, TaskStub and TaskDetails.

partially implements bp async-glance-workers

Change-Id: I2a20b0c5033c6920749370355c2d672dec707c28
Merge "Get rid of TaskDetails in favor of TaskStub"