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.