Add the charging characteristics to the dbi function to allow the
backend to decide the profile based on it.
The charging characteristics can be filled by the HSS and will
be passed by the PCEF towards the PCRF.
This is an API dbi breakage.
Allow database driver to only implement parts of the database driver.
In preprataion of the json interface which will only implement
session_data as might only be used by the PCRF.
The current dbi only supports a single database backend. Add
an abstraction layer to allow multiple database backends.
Introduce a dbi backend struct containing all database functions.
Alexander Couzens