serve: move transfer.advertiseSID check into session_id_advertise()

serve: move transfer.advertiseSID check into session_id_advertise()

In 6b5b6e422ee (serve: advertise session ID in v2 capabilities,
2020-11-11) the check for transfer.advertiseSID was added to the
beginning of the main serve() loop. Thus on startup of the server we'd
populate it.

Let's instead use an explicit lazy initialization pattern in
session_id_advertise() itself, we'll still look the config up only
once per-process, but by moving it out of serve() itself the further
changing of that routine becomes easier.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>