[#3555] Handle conflicts when offer-liftime > 0

[#3555] Handle conflicts when offer-liftime > 0

/src/bin/dhcp4/tests/dora_unittest.cc
    DORATest::reservationsWithConflicts() - added parameter
    for offer-lifetime

    TEST_F(DORATest, reservationsWithConflictsOfferLft)
    TEST_F(DORATest, reservationsWithConflictsOfferLftMultiThreading)
    - new tests

/src/lib/dhcpsrv/alloc_engine.*
    deleteAssignedLease() - new convenience function

    AllocEngine::requestLease4() - added logic to handle conflicting
    lease when offer-lifetime > 0

    AllocEngine::getOfferLft() - added flag to disable DISCOVER
    only check

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