Track client session in DB

[Kangaroux]

• Establish some basic states clients can be in: disconnected, in queue, connected (authd), playing (realmd)
• Update client row in sessions table
• On realmd startup, all rows belonging to that realm in the sessions table should be removed

Self note: if at some point authd/realmd need to communicate in other ways beyond session keys, it probably makes sense to add an API to realmd and drop the sessions table all together. An API opens up possibilities for remote administration, querying (displaying pop on a webpage), etc.