manifest: change manifestctx to not inherit from manifestdict If manifestctx inherits from manifestdict, it requires some weird logic to lazily load the dict if a piece of information is asked for. This ended up being complicated and unintuitive to use. Let's move the dict creation to .read(). This will make even more sense once we start adding readdelta() and other similar methods to manifestctx.

manifest: make one use of _mancache avoid manifestctxs In a future patch we will change manifestctx and treemanifestctx to no longer derive from manifestdict and treemanifest, respectively. This means that consumers of the _mancache will now need to be aware of the different between the two, until we get rid of the manifest entirely and the _mancache becomes only filled with ctxs. This fixes one case of it that can be fixed by using the other cache. Future patches will address the others uses using the upcoming manifestctx.read() function.