flush - abort a message
Tflush tag  oldtag 
Rflush tag 
When the response to a request is no longer needed, such as when a user interrupts a process doing a read, a Tflush request is sent to the server to purge the pending response. The message being flushed is identified by oldtag. The semantics of flush depend on messages arriving in order.
The server must answer the flush message immediately. If it recognizes oldtag as the tag of a pending transaction, it should abort any pending response and discard that tag. In either case, it should respond with an Rflush echoing the tag (not oldtag) of the Tflush message. The response to a Tflush message should never be an Rerror message.
When the client sends the Tflush, it should disregard all messages received with tag oldtag until the corresponding Rflush is received, at which point oldtag may be recycled for subsequent messages.
The following exceptional conditions are handled correctly by the above specification:
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