SYSDB: Open a timestamps cache for caching domains
For all domain types, except the local domain, open a connection to a
new ldb file located at /var/lib/sss/db names timestamps_$domain.ldb.
Constructs the ldb file path manually in sysdb_check_upgrade_02() but
that should be acceptable because nobody should be running such an old
cache these days anyway.
Reviewed-by: Sumit Bose <firstname.lastname@example.org>