Cleanup: use CMM accessors for consumer_quit variable

Use CMM_LOAD_SHARED and CMM_STORE_SHARED, which are strictly
equivalent to a volatile variable, in line with the rest of the
lttng-tools project.

Also move its declaration to a header, rather than having multiple
declarations in C files, now following our coding style.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>