Fix: report error to client on consumerd error

Adds a consumer daemon state flag that allows us to know the state of
the consumer during client command processing.

Please refer to the comment in lttng-sessiond/main.c above the
definition of those flags for more information and a possible race
condition issue.

This is a quick fix for bug #137 that will be merged in 2.0-stable
branch.

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