Add one-shot mode to twine-writerd

[nevali]

Add a mode to twine-writerd which would cause it to run, fed from the queue, until the queue is dry.

Usefulness would depend upon cluster setup and MQ type. For example, in development, using a database-based MQ implementation such as Anansi's or Spindle's, it can be useful to invoke a run of "process all outstanding items until there are no outstanding items".