impl(rest): make the CurlImpl spill buffer circular (#10474)

Eliminate "shift down" logic in the spill buffer and WriteVector.  While
this probably doesn't make a measurable difference ...

```
  //google/cloud:internal_curl_rest_client_integration_test
    old over 100 runs: max = 5.0s, min = 2.6s, avg = 4.6s, dev = 0.3s
    new over 100 runs: max = 5.0s, min = 2.3s, avg = 4.6s, dev = 0.4s
```

..., removing redundant copying is always a good thing.  Fixes #7958.

Move the spill buffer code into its own class so that it might be tested
individually.

Initialize `download_stall_minimum_rate_` (from its option) instead of
overwriting `transfer_stall_minimum_rate_`.

Do not store an entire `Options` object when we only need the value of
the `CurlFollowLocationOption`.

Close #8059 by commenting on how the libcurl API does not support data
buffers owned by the caller.  This means there will always be additional
copying of the request/response data into/from libcurl.

Close #9200 by removing the TODO for an already-answered question.

Close #7957 by removing the TODO for an already-answered question.

Group the `extern "C"` libcurl callbacks, and make them static within
the cc file.  Also immediately convert the libcurl ptr/size pairs to
`absl::Span<char>`.

Cleanup some comments that seem to refer to older versions of the code,
and add some missing includes.  Rename some variables, like `buffer` and
`ptr`, to give a clearer indiction of their roles.  Remove the trailing
newlines from all `GCP_LOG()`ing.

Author: devbww