ikev2: Use hashes to detect retransmits

ikev2: Use hashes to detect retransmits

This way we avoid parsing messages with unexpected message IDs, which
might not even be possible if we don't have the keys anymore.  However,
the next commit should avoid the latter and this way we avoid deriving
keys for retransmits or unexpected messages.

This also changes how retransmits for fragmented messages are triggered.
Previously, we waited for all fragments and reconstructed the message
before retransmitting the response.  Now we only track the first
fragment and if we receive a retransmit of it respond immediately
without waiting for other fragments (which are now ignored).  This is in
compliance with RFC 7383, section 2.6.1, and can avoid issues if there
are lots of fragments.