We don't use reverse listing in any of our code, outside of tests, and
it is only exposed through libuplink in the
lib/uplink.(*Project).ListBuckets() API. We also don't know of any users
who might have a need for reverse listing through ListBuckets().
Since one of our prospective pointerdb backends can not support
backwards iteration, and because of the above considerations, we are
going to remove the reverse listing feature.
Change-Id: I8d2a1f33d01ee70b79918d584b8c671f57eef2a0
0c025fa937