revlog: move censor logic out of censor extension The censor extension is doing very low-level things with revlogs. It is fundamentally impossible for this logic to remain in the censor extension while support multiple storage backends: we need each storage backend to implement censor in its own storage-specific way. This commit effectively moves the revlog-specific censoring code to be a method of revlogs themselves. We've defined a new API on the file storage interface for censoring an individual node. Even though the current censoring code doesn't use it, the API requires a transaction instance because it logically makes sense for storage backends to require an active transaction (which implies a held write lock) in order to rewrite storage. After this commit, the censor extension has been reduced to boilerplate precondition checking before invoking the generic storage API. I tried to keep the code as similar as possible. But some minor changes were made: * We use self._io instead of instantiating a new revlogio instance. * We compare self.version against REVLOGV0 instead of != REVLOGV1 because presumably all future revlog versions will support censoring. * We use self.opener instead of going through repo.svfs (we don't have a handle on the repo instance from a revlog). * "revlog" dropped * Replace "flog" with "self". Differential Revision: https://phab.mercurial-scm.org/D4656