bt2: prepend `_user` to overridable protected methods

It is possible that, in the future, we add new, optional overridable
protected methods that the user can implement within a component class
or message iterator class to have new features.

There is the risk, however, that the user class already has a method
named as such. This would be considered a backward compatibility break
because the same user class with two different versions of Babeltrace 2
would not work the same way (and with the newest version, it probably
would not work at all).

To prevent this, this patch adds the `_user` namespace to all the
overridable protected methods. It will be documented that you cannot,
when you inherit one of the `bt2._User*` classes, have a method named
`_user_*()`, as this prefix is reserved for the future within the `bt2`
package.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I0c5ac29a21a83d6f644d0bb696f71d687ecfebae
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1777
Tested-by: jenkins <jenkins@lttng.org>