Handle slave database connection in EngineFacade

Make it possible to use an additional (aka, 'slave') database
connection. This might be useful for offloading of read operations
to reduce the load on the RDBMS. Currently, this is only used in
Nova, but can be ported to other projects easily.

Change-Id: I4296347bb9ee0743738fe1126d1cef8e43b9f96e
Merge "Handle slave database connection in EngineFacade"