Properly isolate database backend configuration

Properly isolate database backend configuration

While database backends have had private context for some time they've
all been using the same configuration details from the global config
structure. Create a new DB specific config structure and initialise
a single instance from the config file. Also modify the DB backend
initialise functions to take this config structure as a parameter.
This will allow in the future for multiple different backends (whether
the same type or different) to be included at the same time.