Make the consumer sends a ACK after each command

This is needed to avoid buffer bloating when throttling communication
between the consumer and the relayd. Considering a very low bandwith
limit between the relayd and consumerd, the session daemon would send a
high debit of commands to the consumer without ever

emptying the unix socket queue, which makes the UNIX socket reach buffer
full conditions, which is prone to trigger corner-cases behaviors in
blocking send/recv with MSG_WAITALL, which is likely the cause of hang
experienced when limiting relayd bandwidth.

Adding an ACK to each command makes sure that we acknowledge the session
daemon that we, the consumer, have emptied the unix socket buffer.

NOTE: In consumer_add_relayd_socket(), there might be a problem with the
error path and message status to the sessiond. A subsequent patch might
fix a possible issue but for now it is not at all critical since any
critical error on the consumer side will notify the sessiond through the
error socket.

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