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