Refactor job control to use RStream events

Instead of a single 'job read' callback, job control consumers need to provide
callbacks for "stdout read", "stderr read" and "exit". For vimscript, the
JobActivity autocommand is still used to handle every job event, for example:

```vim
:let srv1_id = jobstart('netcat-server-1', 'nc', ['-l', '9991'])
:let srv2_id = jobstart('netcat-server-2', 'nc', ['-l', '9991'])

function JobEvent()
  " v:job_data[0] = the job id
  " v:job_data[1] = the event type, one of "stdout", "stderr" or "exit"
  " v:job_data[2] = data read from stdout or stderr
  if v:job_data[1] == 'stdout'
    let str = 'Message from job '.v:job_data[0].': '.v:job_data[2]
  elseif v:job_data[1] == 'stderr'
    let str = 'Error message from job '.v:job_data[0].': '.v:job_data[2]
  else
    " Exit
    let str = 'Job '.v:job_data[0].' exited'
  endif

  call append(line('$'), str)
endfunction

au JobActivity netcat-server-* call JobEvent()
```

And to see messages from 'job 1', run in another terminal:

```sh
bash -c "while true; do echo 123; sleep 1; done" | nc localhost 9991
```

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