spanner

This is motivated by #5703. Applications may need to persist the state of pagination iterators, for example, because they are implementing a web service that returns pages of results.

I suggested this API:

struct Page {
  std::vector<std::string> bucket_names;
  std::string reader_state;
};

Page BucketsPage(gcs::Client client, std::string reader_state) {
  auto reader = client.

Per @rfk:

Is it necessary to support the X-Weave-Quota-Remaining header? Does client do anything with it?

[rfk] we fire an observer notification but I can't find any uses of it in Desktop, and we have no plans to implement it on Mobile. I'd be happy for it to be rem

What

Package document comment is missing.

It is useful to understand how to use the packages.