sidedata-exchange: rewrite sidedata on-the-fly whenever possible When a A exchanges with B, the difference of their supported sidedata categories is made, and the responsibility is always with the client to generated it: - If A pushes to B and B requires category `foo` that A does not have, A will need to generate it when sending it to B. - If A pulls from B and A needs category `foo`, it will generate `foo` before the end of the transaction. - Any category that is not required is removed. If peers are not compatible, abort. It is forbidden to rewrite sidedata for a rev that already has sidedata, since that would introduce unreachable (garbage) data in the data file, something we're not prepared for yet. Differential Revision: https://phab.mercurial-scm.org/D10032