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This change includes the code implemented by the pull request
submitter as well as Marcin's fixes/changes on top of it, but
without a review.

Also added checks for overlapping pools within a subnet.

Also fixed a couple of failing unit tests.

 - IOAddress::increase() simplified
 - getLeasesCount renamed to getPoolCapacity
 - removed several intermediate variables

 - getRelayInfo() implemented, relay_ is now protected
 - setRelay => setRelayInfo
 - Option::Universe is passed as const reference
 - Further clarified white_list_ field in Subnet.h

 - Copyright years updated
 - Minor corrections in BINDX Guide
 - dhcp4.spec relay structure description updated.
 - new unit-test for DHCPv4 client classification added
 - new unit-test for DHCPv6 client classification added
 - fixed Doxygen errors
 - comments for CfgMgr::getSubnet{4,6} updated

 - new negative unit-test for next-server written
 - comments update (capitalized!)
 - indents corrected
 - no more raw pointers
 - ChangeLog added

 - lease_type_ renamed to pool_type_
 - exception message clarified
 - prefix lengths comment updated
 - couple EXPECT_NO_THROWs added to subnet unit-tests