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.