Fix: don't spawn relayd if URL is provided

In the lttng command line tool, relayd should only be spawned for the
default use case which is no URL provided.

Signed-off-by: David Goulet <dgoulet@efficios.com>
Fix: take session list lock when listing tp

This is important since the list tracepoints command access the
application socket to ask the application for its TPs. The session list
lock protects the ordering of message for those sockets.

This was triggering out of order message between the session daemon and
an application thus triggering undefined behavior.

Fixes #774

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