Take DBAAS multi-channel publishing Redis modules into use

Take DBAAS multi-channel publishing Redis modules into use

Following SDL APIs are defined so that multiple channel-event pairs can be
given as function argument but actual SDL implementation utilized such a DBAAS
(Redis) module what expect to get only one channel-event pair. Fix the
implementation of these SDL APIs to use the correct DBAAS module what support
multiple channel-event pairs:
* set_if_and_publish()
* set_if_not_exists_and_publish()
* remove_if_and_publish()

Please note that in runtime environment DBAAS service needs to run on DBAAS
image version 0.4.0 or newer. Older images do not have multiple channel-event
pairs support as a Redis module.

Updated also start_event_listener() and handle_events() description to
emphasize that channel-event subscription must be done before calling those
functions, otherwise open-source Redis client application could throw an
exception due to not having any subscription when it receives some event, for
example a ping event in case of the DBAAS Sentinel deployment.

Issue-ID: RIC-758

Signed-off-by: Timo Tietavainen <timo.tietavainen@nokia.com>
Change-Id: I75d4e3316d7387b7a1a735fe3eb357de7ef794bb