Kinto-changes

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**Bug fixes**

- Bypass storage when redirecting on old ``_since``

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**New features**

- Redirect clients whose ``?_since`` value is too old (21 days by default)
- Add support for monitor/changes in changeset endpoints (fixes 173)

**Bug fixes**

- Fix validation rule for ``_since`` query parameter

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**Bug fixes**

- Include tombstones in changeset endpoint (fixes 175)

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- Sort changes explicitly by `last_modified` descending

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**Breaking changes**

- Switch changeset timestamp from ETag to integer (168)

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**New features**

- Add changeset endpoint (154)

**Bug fixes**

- Fix crash with ``If-None-Match: *`` (fixes 158)