verify: keep the revlog open for reading while verifying it This is more explicit. No performances difference were observed.

convert: use `changelogrevision` to fetch changeset data instead of read This `changelogrevision` is a higher level function that will remain simple to use as we refactor the revlog.

revlog: adapt the `reading` check for `unionrepo` We cannot just rely on the length check for the `unionrepo` as the local revlog might be empty while the other revlog contains data. In addition, we need to also open the second revlog for reading when needed.

revlog: adapt the `reading` check for `bundlerepo` We cannot just rely on the length check for the `bundlerepo` as the local revlog might be empty with all data in the bundle.

revlog: make `reading` not crash on empty repository If the revlog is empty, the file might not exist and the open will fails. This is not great, but that details or this is now contained in the revlog itself.

changelog: make the `_delayopener` a real proxy We use the same approach as the `_divertopener`. This make it possible to use other methods of the vfs and not just the `__call__` API.

perf: ensure all readlog's reading is done within a `reading` context We are about to enforce this at the revlog level, so we update the perf code in advance.

perf: change the way we approach revlog reading If the `reading` context manager is available, we should use it over explicit file handle management. This will help us to make file handle management a matter more internal to the revlog.

revlog: document the `rev` method A small docstring does not hurt.

revlog: document the `clearcaches` method A small docstring does not hurt.