Implement SDL multi-namespace API

Add 'SyncStorage' with its write and read APIs to support multi-namespace API
what has been implemented already in Python and C++ SDL APIs. In Multi-
namespace API, at SyncStorage' creation time, connection to database backend is
created but namespace is not defined. Namespace is given later as a parameter
to every SDL read and write API calls what makes it easy to write and read from
different namespaces in single SDL instance of type 'SyncStorage'.

This multi-namespace implementation is a prerequisite to have before sentinel
based DB capacity scaling feature RIC-699 can be implemented properly.

Issue-ID: RIC-699

Signed-off-by: Timo Tietavainen <timo.tietavainen@nokia.com>
Change-Id: I018dde1f016630d6b6e44e66a2f276a04e38505a