manifest: support time-based patch history

[This](https://conda.discourse.group/t/how-we-reduced-condas-index-fetch-bandwidth-by-99/257) kind of thing could significantly reduce download requirements for users accessing the store, especially as our number of extensions scales up. Much easier to scale fast if the appropriate infrastructure has been designed in from early on :-)

Patch history is already possible to some extent with the git history of the `gh-pages` branch, especially if we enforce an ordering for the extensions and versions, but a content-aware JSON patch history could likely be a fair amount more efficient.