Assert that the consumer socket lock is taken during communication

Assert that the consumer socket lock is taken during communication

The consumer_data lock must be acquired during any communication
between the session and consumer daemons.

Stress tests have shown a number of deadlocks that have been
traced down to this type of errors.

Individual fixes follow this commit.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>