Rename "tracing session" -> "recording session"

Starting from LTTng 2.13, _tracing_ is defined as attempting to execute
one or more actions when emitting an event, which is very close to the
trigger definition.

To highlight that a tracing session is only about event recording,
rename this concept to _recording session_.

This patch mostly changes the manual pages, although I also updated some
C source and other files which contain user-facing text to use the new
term.

I didn't update logging messages because debugging scripts could still
refer to "tracing sessions".

The lttng-concepts(7) manual page mentions that the "recording session"
term was "tracing session" before LTTng 2.13.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I620d6b6be9e0f1dac14c0bc5e26094c3b3711c75