updatedAt is stamped when a write is built, not when it commits, so a
transaction in flight when the cursor was handed out could commit a row
stamped before it, and a watermark at serverNow skipped that row for
good. The watermarks now sit one minute back; client upserts are
idempotent, so the overlap is harmless. The token's issue time stays
serverNow.
GET /api/sync/changes?start=now answers an empty page whose cursor sits at
serverNow in every domain, in the same opaque format the feed hands out.
A client that only needs what changes from now on, such as the Apple
Health write-back, pages forward from it instead of draining the whole
history first. start and cursor together are refused with 422.
Refs #1339
Marc-André Bombeck