[5007] Suppress DDNS updates on DHCPv6 lease renewals unless the FQDN changes

[5007] Suppress DDNS updates on DHCPv6 lease renewals unless the FQDN changes

src/lib/dhcpsrv/alloc_engine.h
src/lib/dhcpsrv/alloc_engine.cc
    - AllocEngine::extendLease6()
    - AllocEngine::updateLeaseData() - logic was added to clear the context
    DNS update flags when the renewal does not alter the lease's FQDN.

src/bin/dhcp6/dhcp6_srv.h
src/bin/dhcp6/dhcp6_srv.cc
    - Dhcpv6Srv::createNameChangeRequests() - added context as second parameter,
    and modified function to return without creating NCR(s) if both update flags
    in the context are false.

src/bin/dhcp6/tests/fqdn_unittest.cc
    - TEST_F(FqdnDhcpv6SrvTest, createNameChangeRequests) - modified to verify
    combinations of context update flags

    - TEST_F(FqdnDhcpv6SrvTest, DISABLED_processTwoRequests) - renamed
    to TEST_F(FqdnDhcpv6SrvTest, processTwoRequestsDiffFqdn) and enabled.
    It had been disabled pending 3677 which has been completed.

    - TEST_F(FqdnDhcpv6SrvTest, processTwoRequestsSameFqdn)  - new test
    which verifies that client "renewing" a lease by sending a second
    request with the different FQDN generates the correct NCRs

    - TEST_F(FqdnDhcpv6SrvTest, DISABLED_processRequestRenew) - renamed
    to TEST_F(FqdnDhcpv6SrvTest, processRequestRenewDiffFqdn) and enabled.
    It had been disabled pending 3677 which has been completed.

    - TEST_F(FqdnDhcpv6SrvTest, processRequestRenewSameFqdn) - new test
    which verifies that client renewing a lease by sending a renew
    with the same FQDN does NOT generate any NCRs