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.
quote table name

avoids syntax errors from hyphens
Merge pull request #4080 from minrk/task-history

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.