src.ctf.lttng-live: cleanup: rename functions and variables

src.ctf.lttng-live: cleanup: rename functions and variables

Use term "youngest" whenever possible to signify our current best
candidate.

Rename `lttng_live_iterator_next_on_stream()` ->
`lttng_live_iterator_next_msg_on_stream()` to signify that it's
returning a message.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I7dcfccda30c87662209ac87e3bcda31699d3ea44
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1889
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>