lfs: introduce a localstore method for downloading from remote stores The current local.write() method requires the full data, which means concatenating file chunks in memory when downloading from a git server. The dedicated method downloads in chunks, verifies the content on the fly, and creates the usercache hardlink if successful. It can also be used for the file system based remotestore. An explicit division of labor between downloading from a remote store (which should be verified) and writing to the store because of a commit or similar (which doesn't need verification), seems clearer. I can't figure out how to make a similar function for upload, because for a file remote store, it's a simple open/read/write operation. For a gitremote store, it's open the file and a urlreq.request(), and process that.