Rename unix transport identifiers to demystify the package

Three nearly identical names covered three unrelated concepts
(ConnectedEndpoint, connectedEndpoint, connectionedEndpoint), and the
notification callbacks were named for the opposite of what they do
(RecvNotify woke writers, SendNotify woke readers).

- ConnectedEndpoint -> Sender, the write half of a connection
  (implementations queueSender, HostSender, SCMSender); the
  baseEndpoint field becomes connected -> peer: the type is named for
  what you do with it, the field for its role to the holder.
- SendNotify -> NotifyDataReady, RecvNotify -> NotifyWriteSpace,
  CloseNotify -> NotifyStateChange; each named for the wakeup it
  delivers. NotifyStateChange must wake hangup-only pollers.
- queue's embedded waiter queues become readerWaiters/writerWaiters,
  and baseEndpoint's embedded *waiter.Queue becomes a named field,
  WaitQueue, so wakeup call sites name their target.
- writeShutdown and wrShutdown/rdShutdown ->
  sendShutdown/recvShutdown, the shutdown(2) directions they track.
- A package doc block describes how the pieces fit together.

No behavior change.

PiperOrigin-RevId: 966732452