[#1710] Added lifetimes to v6 client classes

kea-dhcp6 client classes now support valid and preferred
lifetime triplets.  Need to update ARM and CB yet.

/doc/examples/kea6/all-keys.json
    Updated

/src/bin/dhcp6/dhcp6_lexer.ll b/src/bin/dhcp6/dhcp6_lexer.ll
    Added preferred and valid lifetime parameters to client class

/src/bin/dhcp6/dhcp6_parser.yy
    Added preferred and valid lifetime parameters to client class

src/bin/dhcp6/tests/config_parser_unittest.cc
    TEST_F(Dhcp6ParserTest, clientClassValidPreferredLifetime)  - new test

/src/bin/dhcp6/tests/parser_unittest.cc
    Added use of EXPECT_NO_THROW_LOG to ease finding JSON errors

/src/lib/dhcpsrv/alloc_engine.*
    AllocEngine::getLifetimes6() - new function
    Lease6Ptr AllocEngine::createLease6() - calls new function

/src/lib/dhcpsrv/client_class_def.*
/src/lib/dhcpsrv/parsers/client_class_def_parser.cc
    ClientClassDef - added support for preferred lifetime triplet

/src/lib/dhcpsrv/tests/alloc_engine6_unittest.cc
    TEST_F(AllocEngine6Test, getValidLifetime)
    TEST_F(AllocEngine6Test, getPreferredLifetime) - new tests

/src/lib/dhcpsrv/tests/client_class_def_parser_unittest.cc
    TEST_F(ClientClassDefParserTest, preferredLifetimeTests) - new test

/src/lib/dhcpsrv/tests/client_class_def_unittest.cc
    updated tests

doc/examples/kea6/all-keys.json

@@ -22,7 +22,25 @@
 
                 // Class selection expression. The DHCP packet is assigned to this
                 // class when the given expression evaluates to true.
-                "test": "member('HA_server1')"
+                "test": "member('HA_server1')",
+
+                // Class valid lifetime.
+                "valid-lifetime": 6000,
+
+                // Class min valid lifetime.
+                "min-valid-lifetime": 4000,
+
+                // Class max valid lifetime.
+                "max-valid-lifetime": 8000,
+
+                // Class preferred lifetime.
+                "preferred-lifetime": 7000,
+
+                // Class min preferred lifetime.
+                "min-preferred-lifetime": 5000,
+
+                // Class max preferred lifetime.
+                "max-preferred-lifetime": 9000
             },
             {
                 // Second class name.