[#13,!6] allocation engine modifications to support v6 global host reservations

[#13,!6] allocation engine modifications to support v6 global host reservations

src/lib/dhcpsrv/alloc_engine.cc
    New functions:
        AllocEngine::ClientContext6::globalHost()
        AllocEngine::ClientContext6::hasGlobalReservation()
        AllocEngine::findGlobalReservation()
        AllocEngine::allocateGlobalReservedLeases6()

    Modified functions:
        AllocEngine::ClientContext6::currentHost() - modified to take
        into account a global host

        AllocEngine::findReservation() - modified to use findGlobalReservation()
        AllocEngine::allocateLeases6() - modified to use allocateGlobalReservation()
        AllocEngine::removeNonmatchingReservedLeases6() - modified to
        retain global reservations

        AllocEngine::extendLease6() - modified to bypass range and client
        mismatch disqualification for global reservations

src/lib/dhcpsrv/tests/alloc_engine6_unittest.cc
    TEST_F(AllocEngine6Test, globalHostDynamicAddress)
    TEST_F(AllocEngine6Test, globalHostReservedAddress)
    TEST_F(AllocEngine6Test, globalHostReservedPrefix)

src/lib/dhcpsrv/tests/alloc_engine_utils.cc
    testStatistics() - changed to test against SUBNET_ID_UNSUSED

src/lib/dhcpsrv/tests/alloc_engine_utils.h
    testStatistics() - changed subnet_id default