http2: remove conn->data use

... but instead use a private alternative that points to the "driving
transfer" from the connection. We set the "user data" associated with
the connection to be the connectdata struct, but when we drive transfers
the code still needs to know the pointer to the transfer. We can change
the user data to become the Curl_easy handle, but with older nghttp2
version we cannot dynamically update that pointer properly when
different transfers are used over the same connection.

Closes #6520

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