"tahoe cp" should use backupdb, in both directions

[warner]

Now that the backupdb seems to be working well for "tahoe backup", it's time to extend "tahoe cp" to use it too.

In the upload direction (tahoe cp LOCAL REMOTE), the backupdb should be used to let us skip a new upload of a file that's already been uploaded. The goal is to allow periodic "tahoe cp LOCAL REMOTE" (with fixed values of LOCAL and REMOTE) to do as little work as possible.

In the download direction (tahoe cp REMOTE LOCAL), the backupdb should also be used, to let us skip a download of a file that's already been downloaded. When a Tahoe file is downloaded and written to local disk, a path+timestamps-to-URI entry should be added to the db. Before downloading a file to local disk, the disk should be checked for an existing file with the same timestamps: if present, and if the URI matches the URI that was going to be downloaded, the download should be skipped.