func (b *AtomicFixedSizeRingBuf) Advance(n int) {
	b.tex.Lock()
	defer b.tex.Unlock()

	b.unatomic_advance(n)
}
func (b *AtomicFixedSizeRingBuf) Advance(n int) {
	b.tex.Lock()
	defer b.tex.Unlock()

	b.unatomic_advance(n)
}
// Advance(): non-standard, but better than Next(),
// because we don't have to unwrap our buffer and pay the cpu time
// for the copy that unwrapping may need.
// Useful in conjuction/after ReadWithoutAdvance() above.