Client implementation for Bug 960878 -- we can avoid the overhead of a new token fetch if we have no local data to upload for the syncing engines, or are doing a download-only sync, and just want to check if there's anything new on the server. This will go a long way toward making short-circuit syncs cheaper, at the cost of increasing client complexity.

This won't make 29.

Some work in progress commits that test that the Sync endpoints are doing the right thing are at: https://github.com/mozilla-services/android-sync/tree/nalexander/bug-968592-expired-tokens/ Of course, the endpoints don't accept expired tokens yet, so this fails. But the tests are useful, and the old data might be useful too.

See also Bug 985007, which should allow to set the token duration in test code.

Re-triaging per https://bugzilla.mozilla.org/show_bug.cgi?id=1473195 Needinfo :susheel if you think this bug should be re-triaged.

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