topic: don't cache topic of e.g. memctx in _topiccache (issue6500) _topiccache exists to make us hit the storage less often when we want to look at topics for any reason. It doesn't make much sense to cache something that is memory-only and is cheap to access however. Caching things like that was also a source of a bug where creating multiple memctx instances in one process would cache topic of the first one and ignore actual content of .extra() of the others. That was happening because the cache is keyed by .rev(), but all memctx instances have the same .rev() = None.