Assert that the consumer lock is held while sending FDs to consumerd

Assert that the consumer lock is held while sending FDs to consumerd

The consumer_data lock must be held during the communications
between the consumerd and sessiond.

The consumer_data lock is refered-to by each consumer_socket
instance; they point to their consumer's global data lock.

The lock can't be taken in consumer_send_msg() or consumer_send_fds()
since we want to protect a complete "transaction". Some commands
require both functions to be called and we want to hold the lock
over the duration of both calls to protect against other
threads initiating a communication between the two calls.

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