Remove use of channels from p9.connState legacy transport.
Remove use of channels from p9.connState legacy transport.

- Remove sendDone, which currently does nothing whatsoever (errors sent to the
  channel are completely unused). Instead, have request handlers log errors
  they get from p9.send() inline.

- Replace recvOkay and recvDone with recvMu/recvIdle/recvShutdown. In addition
  to being slightly clearer (IMO), this eliminates the p9.connState.service()
  goroutine, significantly reducing the overhead involved in passing connection
  receive access between goroutines (from buffered chan send/recv + unbuffered
  chan send/recv to just a mutex unlock/lock).

PiperOrigin-RevId: 327476755

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