New object: filter-set
defines a set of routes that are matched by its filter
filter-set: [mandatory] [single] [primary/look-up key]
descr: [mandatory] [multiple]
filter: [mandatory] [single]
remarks: [optional] [multiple]
tech-c: [mandatory] [multiple] [inverse key]
admin-c: [mandatory] [multiple] [inverse key]
notify: [optional] [multiple] [inverse key]
mnt-by: [mandatory] [multiple] [inverse key]
changed: [mandatory] [multiple]
source: [mandatory] [single]