src.ctf.lttng-live: Extract `handle_late_message()` function for future fix

src.ctf.lttng-live: Extract `handle_late_message()` function for future fix

An upcoming commit will include a fix regarding the handling of late
messages. So, extract this logic into its own function simplifies the
that future commit.

No change of behaviour intended by this commit.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Ida4f980ab2574ebc64fb455e22047233c0f25cee
Reviewed-on: https://review.lttng.org/c/babeltrace/+/6697
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>