[#1781] HA now load balances only specific message types

[#1781] HA now load balances only specific message types

Added a ChangeLog

src/hooks/dhcp/high_availability/query_filter.*
    QueryFilter::isHaType(const dhcp::Pkt4Ptr& query4)
    QueryFilter::isHaType(const dhcp::Pkt6Ptr& query6)
    - new functions

    QueryFilter::inScope() - uses new functions to balance
    and scope messages of specific types

src/hooks/dhcp/high_availability/tests/query_filter_unittest.cc
    TEST_F(QueryFilterTest, loadBalancingHaTypes4)
    TEST_F(QueryFilterTest, loadBalancingHaTypes4MultiThreading)
    TEST_F(QueryFilterTest, loadBalancingHaTypes6)
    TEST_F(QueryFilterTest, loadBalancingHaTypes6MultiThreading)
    - new tests

src/lib/dhcp/dhcp6.h
    Uncommented v6 message types.