Fix: sessiond: acquire session list lock when updating event notifiers

Registering triggers with an on-event hit condition affects event
notifiers, imposing a synchronization of enablers with the user space
tracers.

As noted in the comments of session.h, the session list lock protects
those updates and is, ultimately, ill-named. The comment is adjusted to
mention "tracer configurations" rather than "session configurations"
since event notifiers are not part of a session, making the comment
imprecise.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Id1bcbcccbdeeafa91176ed3413ddddbcbab10ad2