stream: discard incoming data overlapping unread offset
stream: discard incoming data overlapping unread offset

When an incoming data chunk with an offset lower than the lowest offset
that is yet to be read by the application but with maximum offset higher
than that, is queued into RecvBuf, the RecvBuf::ready() method would be
unable to ever return true, so no matter how much data was received, it
wouldn't be returned to the application.

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