51 - spi collisions

It seems that we now have the same understanding of
this issue. I asked Tero to write some text for an
implementation note. Here goes:

Add to the end of Section 4.4 an indented implementation
note as follows:

    Implementation note: In MOBIKE (and also in IKEv2) it is possible
    to have cases where the SPI, protocol and destination address
    may not be uniquely distinguish SA to be used for outgoing
    traffic. In those cases implementations need to have a direct
    pointer from the matched traffic selectors of the outgoing traffic
    to the outbound SA to be used. That is, it is not enough to store SPI,
    protocol, and destination address to the SPD-S, but some kind of
    unique identifier is also needed. 

--Jari