CookieNameChange allowed the use of the expand-contract pattern for
cookies, but the suggested procedure assumed an instantaneous change on
the server side. Without that, there would be a brief window of time where
servers that had received the newer config would be writing cookies that
the servers with old config would not be able to understand.
However, the mechanism can be made seamless by using it *twice* in
succession, with the first usage in a "no-op" configuration. This allows
all the servers to become aware of the new name without using it. The
second change flips the configuration and changes the official name of the
cookie, and during that window both sets of servers are able to understand
both sets of names, even though they're sending a mix of names. This can
then be followed by the usual cleanup.
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