ben-manes / caffeine / #5695

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Report the timer wheel's next bucket flush as its delay

The expiration delay is when the wheel next flushes a bucket holding a
timer, which either expires the entry or cascades it towards a finer
resolution. A bucket is flushed once the wheel reaches its tick, so the
current bucket, whose tick has already passed, is flushed by the next
one rather than a full span later.

The scan reported a whole span when the current bucket was occupied and
returned before probing the rest of that wheel, hiding an event a single
bucket ahead which then fired up to a span late. The current bucket is
occupied because findBucket chooses the wheel by duration but the index
by absolute time, so a duration within the clock's offset into the span
of the next span's start wraps a full revolution onto it. That entry is
a revolution out while the one it masks is imminent.

peekAhead skipped the higher wheel's current bucket, which is flushed
alongside the one it probed. Only wrapped timers land there, so nothing
expired late, but the delay no longer described the flush that followed
it.

The fuzzy test now asserts the delay equals the earliest flush across
every bucket. Its previous assertion was guarded by the same condition
it asserted and compared an absolute time against a relative delay.

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