bt2: Mass notification -> message rename

As the "notification" concept has been renamed to "message", I decided
to do a mass rename (including renaming files).  It's quite
straightforward and will reduce a little bit the noise in further
patches.

The changes were done mostly mechanically, but I verified them to make
sure it didn't go too wild (like changing "notifier" to "msgier", which
we wouldn't want).

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I6e6dec1234864400a2570a4726c32a1398cf42c0
Reviewed-on: https://review.gerrithub.io/c/eepp/babeltrace/+/452232
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: Philippe Proulx <eeppeliteloop@gmail.com>