backupdb: add directory cache

[warner]

We can speed up the #598 "tahoe backup" command even more by caching tahoe-side read-only directory contents. At the moment, we have to read every Tahoe directory to find out whether we can share it with a new backup or not. With a table in the backupdb that maps dircap to hash of its contents, we can avoid those reads, and get rid of almost all of the network IO involved in a null backup.

[zooko]

If you like this ticket, you might also like #465 (add a mutable-file cache), #316 (add caching to tahoe proper?), and #300 (macfuse: need some sort of caching).

[warner]

now that we've got immutable dirnodes, and I'm working on using them in "tahoe backup" (#828), I want the 1.6.0 release to include this feature, so that "tahoe backup" will be super-efficient.

[warner]

Done, in 9901a08fd30efa87.