Clarify and expand some tests, and add a large test that tries to cover
the whole segment-verify stack.
Trying to reproduce a problem we saw in production where the Found count
in the output csv increases with each entry, eventually reaching the
thousands, instead of representing the actual number of pieces found for
each entry.
Change-Id: I65e342fad7dc1c350830fd0c8ce75a87a01d495c
* Disallow too large listing limit, which would cause a lot of memory to
be consumed.
* Fix throttling logic and add a test.
* Fix read error handling; depending on the concurrency it can return
the NotFound status either in the Read or Close.
Change-Id: I778f04a5961988b2480df5c7faaa22393fc5d760
