[#3463] Implementd leases4_committed callout

[#3463]  Implementd leases4_committed callout

/src/hooks/dhcp/lease_cmds/binding_variables.cc
    BindingVariableMgr::evaluateVariables()
    - Add a throw if query,response, or lease are empty

/src/hooks/dhcp/lease_cmds/lease_cmds.*
    LeaseCmdsImpl::leases4Committed()
    LeaseCmds::leases4Committed()
    - implemented callout handler

/src/hooks/dhcp/lease_cmds/lease_cmds_callouts.cc
    leases4_committed(CalloutHandle& handle) - new callout

/src/hooks/dhcp/lease_cmds/lease_cmds_messages.mes
    LEASE_CMDS_LEASES4_COMMITTED_NOTHING_TO_UPDATE
    LEASE_CMDS_LEASES4_COMMITTED_FAILED processing - new messages

/src/hooks/dhcp/lease_cmds/tests/lease_cmds4_unittest.cc
    void Lease4CmdsTest::testValidLeases4Committed()
    TEST_F(Lease4CmdsTest, validLeases4Committed)
    TEST_F(Lease4CmdsTest, validLeases4CommittedMultiThreading)
    - new tests