[#2719] kea-dhcp4 offer-lft is now functional

[#2719] kea-dhcp4 offer-lft is now functional

kea-dhcp4 supports offer-lft for global, shared-network,
and subnets.  Not yet supported in classes.

src/bin/dhcp4/dhcp4_srv.cc
    Dhcpv4Srv::assignLease() - adjust outbound lease time option

    Dhcpv4Srv::postAllocateNameUpdate() - allow reuse check on allocated
    discover

src/lib/dhcpsrv/alloc_engine.*
    AllocEngine::ClientContext4 - add offer_lft_ member
    AllocEngine::discoverLease4() -set contexts offer_lft_
    AllocEngine::getOfferLft() - new, get context appropriate value for offer_lft
    AllocEngine::createLease4() - use offer_lft on DISCOVER, pass offer_lft
    into hook, set lease fqdn flags to false
    AllocEngine::reuseExpiredLease4() - reclaim on discover allocation
    AllocEngine::updateLease4Information() - use offer_lft if appropriate

src/lib/dhcpsrv/tests/alloc_engine4_unittest.cc
    TEST_F(AllocEngine4Test, discoverOfferLft) - enabled
    TEST_F(AllocEngine4Test, discoverOfferLftReuseExpiredLease4) - new test