Make DB instance visible

Make DB instance visible

Make underlying DB instance visible to clients. This change makes it
easier for the clients to e.g. share the same DB instance between
multiple SDL instances (multiple namespaces is used). The DB instance
can be created once and reuse it every time a new SDL instance is
created. The DB instance is created with Newdatabase() method.

Update the documentation so that it is mentioned that subscribing the
channels is not safe for concurrent usage.

Change-Id: I1b09500eda44b6b095a3bf564c38e794d02f9d95
Signed-off-by: Marco Tallskog <marco.tallskog@nokia.com>