Don't change a session once its in the cache
Don't change a session once its in the cache

Sessions should be immutable once they are in the cache because they could
be shared with other threads. If you change them then this can cause
corruptions and races

Reviewed-by: Rich Salz <c04971a99e5a9ee80eaab4b1deb37e845b0bd697@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6561)

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