[#821,!501] kea-dhcp4 now sanity checks inbound messages

[#821,!501] kea-dhcp4 now sanity checks inbound messages

src/bin/dhcp4/dhcp4_srv.cc
    Dhcpv4Srv::processRequest()
    Dhcpv4Srv::processRelease()
    Dhcpv4Srv::processDecline()
    Dhcpv4Srv::processInform() - now all call sanityCheck()

src/bin/dhcp4/tests/dhcp4_srv_unittest.cc
    TEST_F(Dhcpv4SrvTest, sanityCheckDiscover)
    TEST_F(Dhcpv4SrvTest, sanityCheckRequest)
    TEST_F(Dhcpv4SrvTest, sanityCheckDecline)
    TEST_F(Dhcpv4SrvTest, sanityCheckRelease)
    TEST_F(Dhcpv4SrvTest, sanityCheckInform) - new tests

src/lib/testutils/gtest_utils.h
    New file with handy new test macros:
        EXPECT_THROW_MSG()
        ASSERT_THROW_MSG()

src/lib/testutils/Makefile.am
    Added new file gtest_utils.h

Added a ChangeLog entry