cloud backend uses losts of expensive LIST requests

[cloud_trouble]

The cloud backend uses lots of expensive LIST requests with an Amazon S3 bucket from heavy use of GET Bucket. The GET Bucket request is billed as a LIST request and is 10 times more expensive than a GET Object request.

These LIST requests can be a large portion of the cost of using an S3 backed storage node. For example, my logs show 1.5 times as many GET Bucket requests as GET Object requests (with two storage nodes, one S3 bucket and one desktop computer) and the cost exceeds storage, transfer, and ec2 costs.

Here is some relevant code: ​https://github.com/LeastAuthority/tahoe-lafs/blob/cloud-rebased/src/allmydata/storage/backends/cloud/cloud_common.py#L426

And relevant chat on IRC:

<daira1> the list of shares is stored in a local database called the leasedb. that was added recently on the cloud branch, so I suspect we're not making optimal use of it yet <daira1> ISTR that zooko was arguing for treating the leasedb as authoritative as to whether a share exists, and I was arguing against for a reason that I can't remember right now. there's a ticket about it <zooko> Yes, the arguments about the trade-offs of treating leasedb as authoritative vs. advisory are encoded into tickets. <zooko> I seem to recall that treating leasedb as authoritative gets nice performance, including for this particular aspect, while trading off some other values.