async.clj

  * `:on-close` - `(fn [ch {:keys [reason code message]}] ...)` -
    called for *any* close, including a call to [[close]], but will only
    be invoked once.

    `reason` will be one of:

      * :complete - the channel was closed normally
      * :gone - the client disappeared
      * :error - an error occured that forced closure
      * :shutdown - the client or server is shutting down (WebSocket only)
      * :other - some other unknown reason (WebSocket only)

    `code` will be the numeric closure code if the channel is a
     WebSocket, and can be inspected if you need more granularity
     that provided by `reason`.

    `message` will be the optional text for the closure.