Fix: sessiond: don't allocate buffers and files for inactive sessions

When tracing is inactive (before start/after stop), the current behavior
is to track all applications registered as UST data producers and
allocate buffers and files.

However, we guarantee that the trace is readable (invariant) after a
"stop" command has waited for data pending to complete. Unfortunately,
tracking additional applications (and adding their files) after tracing
is stopped (for each pid in per-pid buffers, for new uid in per-uid
buffers) does not respect this guarantee.

Fix this by *not* allocating channels, events, contexts when tracing
is inactive, but rather allocate those lazily just before tracing
starts.

One reason why this was not originally done was to ensure we could
have a fast start command. There are however other ways to achieve this
in the future that will respect the stop invariant guarantees.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>