db_hlr.c: add db_subscr_exists_by_imsi()

Check if a subscriber exists without generating an error log entry if
it does not. This is cheaper than db_subscr_get_by_imsi(), as it does
not fetch the subscriber entry. subscriber-create-on-demand will use
this function.

Related: OS#2542
Change-Id: I63818c0dd4fd22b41dadeeba2a07a651b5454c54
5 files changed