sidedata: move to new sidedata storage in revlogv2
The current (experimental) sidedata system uses flagprocessors to signify the
presence and store/retrieve sidedata from the raw revlog data. This proved to be
quite fragile from an exchange perspective and a lot more complex than simply
having a dedicated space in the new revlog format.
This change does not handle exchange (ironically), so the test for amend - that
uses a bundle - is broken. This functionality is split into the next patches.
Differential Revision: https://phab.mercurial-scm.org/D9993
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