database: Allocate subnets list only once

database: Allocate subnets list only once

This is a performance improvement when exporting networks flattened. For
the subnet search, we allocate an empty list many times which is often
not required.

This patch changes this behaviour that (if needed) the lists will be
allocated and will stay around and cleared if necessary.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>