[#1155] Backported #1139 from master

[#1155] Backported #1139 from master

This is an enhancement to client classification in DHCPv4
and DHCPv6 servers. The client classes spefified in the
host database are taken into account in evaluation of the
client classes specified within the configuration files.
This works both for the global reservations and for the
non-global reservations when the selected subnet belongs
to a shared network. It can be used to influence subnet
selection within a shared network using host reservations
or pool selection within a subnet that doesn't necessarily
belong to a shared network.