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.