[quicwg/base-drafts] Don't change CID on peer CID change (#2145)

[ekr <[e-mail]>]

```Endpoints that use connection IDs with length greater than zero could have their
activity correlated if their peers keep using the same destination connection ID
after migration. Endpoints that receive packets with a previously unused
Destination Connection ID SHOULD change to sending packets with a connection ID
that has not been used on any other network path.  The goal here is to ensure
that packets sent on different paths cannot be correlated. To fulfill this
privacy requirement, endpoints that initiate migration and use connection IDs
with length greater than zero SHOULD provide their peers with new connection IDs
before migration.

Caution:

: If both endpoints change connection ID in response to seeing a change in
  connection ID from their peer, then this can trigger an infinite sequence of
  changes.
```

I don't remember this being what we agreed, and it's not necessary. You only need to change when you see a new path, not after a new CID. That's how we got rid of counting to infinity.

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