Implement summon request protocol

[vswarte]

Rework the spawn procedure a bit:

• In case of a client we should request the host to spawn a summon for us.
• In case of a host we should listen for these requests and make the decision on whether or not this is a valid request.
• In case of a host we should spawn the summons and broadcast SummonSpawned to all clients.
• In case of a host we should never handle SummonSpawned messages as these are meant to inform clients of a new summon.
• In case of a client we should never handle SummonSpawned from any other lobby member than the host.

[vswarte]

Most of the work here has been done except for some anti-abuse items. I still need to:

• Implement role validation for incoming messages (host/client validation)
• Gracefully handle incorrect statemachine transitions (just drop them? stop handling messages for users that seem abusive?)