SYSDB: Add a second, timestamp-only ldb cache
    
    Uses the generic functions in sysdb_init.c to open a new ldb database.
    The path to the database and the ldb context are stored in the
    sysdb_ctx. The database will be used to store ephemeral attributes such
    as timestamps. Because these attributes are not required for SSSD
    operation and the intent is for writes to this cache to be very fast,
    the database is opened with LDB_FLG_NOSYNC flag. At the same time, none
    of the attributes of the cache is required for sssd operation, so if we
    fail to open the database, we just start over.
    
    Adds a separate base LDIF with attributes that are supposed to be
    indexed in the timestamp database as well as a separate timestamp cache
    version.
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>