Pack notifications to be compatible with SDL golang

In SDL Golang 'publish' API implementation packs all the events to single DB
notification, where events are separated by '___' characters. In SDL Golang
subscriber receives this packed DB notification and it splits the DB
notification by '___' characters and calls application notification handler
callback function with list of received events.
Current implementation of SDL Python is however different, is does not do any
packing for events and hence it calls application notification callback many
times, once for each event it has received. Also if SDL Python application is
used as event subscriber and SDL Golang application as event published,
Python application won't be able to handle those notifications what are packed
to one db notification by SDL Golang application.
With this commit implement notification packing to SDL Python.

Issue-ID: RIC-795

Signed-off-by: Timo Tietavainen <timo.tietavainen@nokia.com>
Change-Id: Ie494430cf46756ea476b98343a8c651a2fa1dbcd