res_calendar: delete old calendars after reload

When "fetch_again_at_reload" is set in config, we create now
new object and thread for each reloaded calendar (with new
configuration). Old calendar should be then unlinked, so the
old thread can exit and free memory.

ASTERISK-26683

Change-Id: Ic17fba9371c5a8b26a6bc54ea4957c13a32a343e