Fix: session lock use after free

The session lock is broken in that it does not handle teardown correctly
(use after free). Surround each usage by the session list lock for now
to fix this issue, and don't unlock the session lock after free. Since
each session lock usage is surrounded by session list lock, no other
thread will be left waiting on this lock when the session destroy is
performed.

This effectively renders useless the per-session lock. Leave it there
for now to minimize code change before 2.0 final.

This locking scheme will be revisited for lttng 2.1.

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