Session cookie can be set twice after session id changed

[janbartel]

jetty 9,10,11

When Request.changeSessionId() is called, the isIdChanged flag on the Session is used to indicate that a new Set-Cookie should be generated with the new session id. However, this flag is not cleared after the cookie is successfully generated. Thus, on the next request, the Set-Cookie header will be generated again. The flag is then cleared correctly. Note that both Set-Cookie headers will be for the same session id - in other words, another session id change is not triggered, just a repetition of the same cookie session id. This only happens when using the DefaultSessionCache, as the same session object is not retained by the NullSessionCache.

[janbartel]

Fixed via #7808 and #7809.