Fix: consumerd errors on exit

- The LTTNG_CONSUMER_STOP command is not used by sessiond anymore, so
  deprecate it,
- Ensure that the normal teardown (with signal handler) does not trigger
  a health error in consumer daemon by distinguishing between errors and
  "should exit" with different lttng_consumer_poll_socket() return values.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>