Added drain functionality to the Sequencer

[accelerated]

Signed-off-by: Alexander Damian adamian@bloomberg.net

Describe your changes
Added a drain() member function to the Sequencer class which is identical in signature to Dispatcher::drain(). While draining can be done directly from the dispatcher object itself, there are situations when the jobs enqueued in the Sequencer may post additional sub-jobs in the Dispatcher. If the Dispatcher drains, these jobs would fail since posting is disallowed while draining. By draining the Sequencer we allow all jobs to complete successfully.

Testing performed
Ran unit tests and replaced Dispatcher::drain() with Sequencer::drain()