MEDIUM: log/session: handle embryonic session log within sess_log()

MEDIUM: log/session: handle embryonic session log within sess_log()

Move the embryonic session logging logic down to sess_log() in preparation
for log-profiles because then log preferences will be set per logger and
not per proxy. Indeed, as each logger may come with its own log-profile
that possibly overrides proxy logformat preferences, the check will need
to be performed at a central place by lower sending functions.

To ensure the change doesn't break existing behavior, a dedicated
sess_log_embryonic() wrapper was added and is exclusively used by
session_kill_embryonic() to indicate that a special logging logic must
be performed under sess_log().

Also, thanks to this change, log-format-sd will now be taken into account
for legacy embryonic session logging.