Ensure GenServer casts complete before continuing execution

[starcraft66]

We have many places in the code base like:

lanparty-seating/lib/lanpartyseating/logic/reservation_logic.ex

Line 146 in d91efb3

GenServer.cast(:"expire_reservation_#{res.id}", :terminate)

which directly fire calls to GenServer.cast/2 which only guarantees that the message was fired off but not that the task has actually done its job.

We should refactor these calls (cc: @berbiche)