Deprecate enable/disable-consumer

The concept of consumer should change to the fact that a user can
enable/disable outputs such as file or network for streaming. We
envision that multiple outputs can be attached to a channel in the
future.

We want to minimise the amount of puzzlement for users. Having an error
message from the consumer daemon when --no-consumer has been specified
can be considered puzzling.

This calls for "add output" semantic, so we can support this "tee"
use-case seemlessly. We plan on allowing add/del output while tracing is
active, which is not currently permitted by enable-consumer.

Everything enable-consumer does can be done by lttng create options
Therefore, it seems easier for development of upcoming features and for
the sake of simplicity of interfaces to deprecate the current
enable/disable consumer.

This applies for both to the lttng UI commands and API calls. The
documentation has been removed from the man page and the help of the
lttng command. So, it exists but is simply not documented.

Signed-off-by: David Goulet <dgoulet@efficios.com>