clean up StreamWriter factory lifecycle

Right now, StreamWriter and children have different lifecycles in continuous processing and microbatch mode. Both execution modes impose significant constraints on what that lifecycle must be, so the achievable consistent semantic is:

• StreamWriter lasts for the duration of the query execution
• DataWriterFactory lasts for the duration of the query execution
• DataWriter (the task-level writer) has a lifecycle tied to each individual epoch

This also allows us to restore the implicit semantic that DataWriter.commit()/abort() terminates the lifecycle.