Add the ability to use a URI for DB connection info

[zerebubuth]

At the moment, all the parts of the database connection information are supplied in the config file separately. #335 extended the config to allow lists of hostnames, and we previously supported lists of database names, for load-balancing purposes. As @iandees pointed out, it would be good to generalise this to allowing lists of arbitrary connection URIs, so that this is unified and extends to all properties.

Will need to consider if/how we want to allow overrides for sensitive information from a different configuration source. E.g: so that the password isn't in the config file.