[ip6] enhance handling of IPv6 fragmentation (#4940)

This commit introduces a few enhancements:

- In NCP architecture forward the IPv6 fragments instead of the full
  reassembled IPv6 packet (which in the default configuration can't be
  forwarder due to SPINEL limitations). Without this fix the
  fragmentation does not work if Thread device wants to send more than
  MTU data through the Border Router (to the IPv6 address of NCP).

- Use OPENTHREAD_CONFIG_IP6_MAX_ASSEMBLED_DATAGRAM only for the
  receiving path. For transmitting path, relay on the message buffers
  pool. This is especially important for some of the use-cases we
  have, and also reduce a need for regenerating OpenThread libraries.

- In case receiving IPv6 packet is bigger than supported
  OPENTHREAD_CONFIG_IP6_MAX_ASSEMBLED_DATAGRAM, exit before trying to
  allocate the next message buffer.

tests/scripts/thread-cert/test_ipv6_fragmentation.py

@@ -77,9 +77,6 @@ class TestIPv6Fragmentation(thread_cert.TestCase):
         self.simulator.go(5)
         self.nodes[LEADER].udp_check_rx(1831)
 
-        self.nodes[ROUTER].udp_send(1953, mleid_leader, common.UDP_TEST_PORT,
-                                    False)
-
         self.nodes[ROUTER].udp_stop()
         self.nodes[LEADER].udp_stop()