change default behavior of database task storage

The previous default behavior was to create a new table (sqlite) / database (mongodb) for each Hub instance, so that task history would not be shared by subsequent IPython sessions.

I don't think this was the right choice, so this PR switches the default behavior to use the table / database `ipython-tasks`. The result is that all Hub sessions in a given profile will share history by default, which makes the benefit of the db backends clearer.

Configurable behavior remains the same, only the default is changed.