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Draft: Associate last DataHeaderForms to all DataHeader objects w/o relying on DHF token

In the past some issues with robustness of the token information linking the DataHeader to its DataHeaderForm. With '_p6' versions, a DataHeaderForm is written only when changes occur and its Token is added to the DataHeader, after both are registered for writing, but before data is stored on disk. This cycle is fragile in client/server cases or with non-ROOT technologies. However, since the information in DataHeaderForm is accumulated, the last DataHeaderForm can be used to decode all DataHeader. The placement of the DataHeaderForm is fixed relative to the DataHeader itself and therefore the Token to the last DataHeaderForm can be determined. This MR will no longer use the DataHeaderForm Token store as part of the DataHeader, but use the last DataHeaderForm in the corresponding container. The MR changes reading of all data, but won't affect writing, in particular, the DataHeaderForm Token is still attached to the DataHeader.

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