Use the prefix "dbping." in the name of internal variables

[cvarela]

It is necessary to rename the following internal variables to use the prefix "dbping." in their name analogously to #5:

• parent.name --> dbping.parent.name
• now --> dbping.now
• task.name --> dbping.task.name
• datasource --> dbping.datasource

To maintain backward compatibility, both variable names should be kept and a warning message should be displayed when the use of variables without the prefix is detected.