Allow parallel database upgrade streams

The initial database upgrader was written to support only the main osTicket database upgrade stream. This expands the upgrader to be able to upgrade not only main stream, but also custom streams or streams from the future plugin architecture.

**NOTE** This request includes the namespaced configuration (feature/federated-config), which re-architects the %config table

**NOTE** This patch starts from `d959a00e`, which is somewhat disheveled at the moment