dnsdist: Add support for range-based lookups into a Key-Value store

dnsdist: Add support for range-based lookups into a Key-Value store

This feature allows doing a range-based lookup (mostly useful for IP addresses), assuming that:
- there is a key for the last element of the range (2001:0db8:ffff:ffff:ffff:ffff:ffff:ffff for 2001:db8::/32)
which contains the first element of the range (2001:0db8:0000:0000:0000:0000:0000:0000) followed by any data in the value
- AND there is no overlapping ranges in the database !!

This requires that the underlying store supports ordered keys, which is true for LMDB but not for CDB, for example.