parser: Make sure that each comparator matcher is a host matcher.

parser: Make sure that each comparator matcher is a host matcher.

So far, 'NOT (service|attribute).<attr> <cmp> <value>' matches did not work
correctly, because the object matcher was not correctly wrapped into a host
matcher. Since we do not currently support querying any object types besides
hosts, this was wrong.

The respective code has been moved into sdb_store_matcher_parse_cmp() which
also implements and checks the restriction on matching host objects only.

A new set of tests has been introduced which exposed this problem and which
(roughly) verifies the result of a lookup using a parsed matcher.