[#1307] kea-dhcp6 supports parked-packet-limit

src/bin/dhcp6/dhcp6_lexer.ll
src/bin/dhcp6/dhcp6_parser.yy
src/bin/dhcp6/json_config_parser.cc
src/lib/dhcpsrv/parsers/simple_parser6.cc
doc/examples/kea6/all-keys.json
    Added parked-packet-limit

src/bin/dhcp6/dhcp6_messages.*
    DHCP6_HOOK_LEASES6_PARKING_LOT_FULL

src/bin/dhcp6/dhcp6_srv.cc
    Dhcpv6Srv::processDhcp6Query() - drops the packet if the parking
    lot is full

src/bin/dhcp6/tests/get_config_unittest.cc
    updated

doc/examples/kea4/all-keys.json

@@ -505,7 +505,7 @@
         // DHCPREQUESTs) that may be parked while waiting for hook library
         // work to complete, prior to a response (e.g. DHCPACK) being sent
         // back to the client.  A typical example is when kea-dhcp4 parks a
-        // DHCPREQUEST while it sends the requesite lease update(s) to its
+        // DHCPREQUEST while it sends the requisite lease update(s) to its
         // HA peer(s). The packet is unparked once the update(s) have been
         // acknowledged.  This value limits the number of packets that can
         // be held pending the updates. In times of heavy client traffic,

doc/examples/kea6/all-keys.json

@@ -503,6 +503,18 @@
             }
         ],
 
+        // Global value which limits the number of client packets (e.g.
+        // REQUESTs,RENEWs...) that may be parked while waiting for
+        // hook library work to complete, prior to a response (e.g. REPLY)
+        // being sent back to the client.  A typical example is when kea-dhcp6
+        // parks a REQUEST while it sends the requisite lease update(s) to its
+        // HA peer(s). The packet is unparked once the update(s) have been
+        // acknowledged.  This value limits the number of packets that can
+        // be held pending the updates. In times of heavy client traffic,
+        // this value can keep kea-dhcp6 from building an insurmountable
+        // backlog of updates.
+        "parked-packet-limit": 128,
+
         // Global (default) value of the preferred lifetime.
         "preferred-lifetime": 50,