tcp: Create new conversations on a SYN after a RST or FIN

If we get a SYN packet with the same sequence number as the current
conversation, but after a RST (or FIN) segment, create a new
conversation.

In such a case, this is probably a peer using the same sequence
number to retry a handshake that failed with a RST due to a half-open
connection. The RST (or ACK that caused a RST) would have an
out-of-window sequence number (or inapplicable ACK for the rest of
the conversation), which can disrupt the follow info, sequence analysis,
desegmentation, etc. unless we create a new conversation.

It could also, less likely, be a new connection after a connection close
that happened to reuse the same sequence number, in which case we also
want to clear out our conversation state.

If we haven't received a RST (or FIN), then consider it a retransmission.

Fix #18333 (also handles the cases of #16944 and #17616 more smoothly).

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