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.

IPython/parallel/controller/mongodb.py

@@ -39,7 +39,7 b' class MongoDB(BaseDB):'
         necessary if the default mongodb configuration does not point to your
         mongod instance."""
     )
-    database = Unicode(config=True,
+    database = Unicode("ipython-tasks", config=True,
         help="""The MongoDB database name to use for storing tasks for this session. If unspecified,
         a new database will be created with the Hub's IDENT.  Specifying the database will result
         in tasks from previous sessions being available via Clients' db_query and

IPython/parallel/controller/sqlitedb.py

@@ -96,7 +96,7 b' class SQLiteDB(BaseDB):'
     location = Unicode('', config=True,
         help="""The directory containing the sqlite task database.  The default
         is to use the cluster_dir location.""")
-    table = Unicode("", config=True,
+    table = Unicode("ipython-tasks", config=True,
         help="""The SQLite Table to use for storing tasks for this session. If unspecified,
         a new table will be created with the Hub's IDENT.  Specifying the table will result
         in tasks from previous sessions being available via Clients' db_query and