implement queue for guests in server

[arades79]

Currently if there are no counselors available when a guest signs in they will get an immediate error and will have to manually try again.

It would be a much more pleasant UI for guests to be able to join a queue when all counselors are taken, and have the server also give them their current place in line.

Along with this, servers should be able to define a maximum queue length such that it never becomes unmanageable.

Likely the queue could also be disabled when no counselors are online - but that may just happen client-side, as number of online counselors is already reported in the probe/info call.