Enumeration RoomKeyRequestState

possible states for a room key request

The state machine looks like:

    |         (cancellation sent)
    | .-------------------------------------------------.
    | |                                                 |
    V V       (cancellation requested)                  |
  UNSENT  -----------------------------+                |
    |                                  |                |
    |                                  |                |
    | (send successful)                |  CANCELLATION_PENDING_AND_WILL_RESEND
    V                                  |                Λ
   SENT                                |                |
    |--------------------------------  |  --------------'
    |                                  |  (cancellation requested with intent
    |                                  |   to resend the original request)
    |                                  |
    | (cancellation requested)         |
    V                                  |
CANCELLATION_PENDING                   |
    |                                  |
    | (cancellation sent)              |
    V                                  |
(deleted)  <---------------------------+
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