performance is not expected for prefetch reader

[lance6716]

In current code we don't fully make use of the prefetch buffer. There's a background goroutine (spawned in line 48) that uses the prefetch buffer to read data from reader (line 57)

tidb/pkg/util/prefetch/reader.go

Lines 48 to 63 in b7e9769

	go ret.run()
	return ret
	}
	func (r *Reader) run() {
	defer r.wg.Done()
	for {
	r.bufIdx = (r.bufIdx + 1) % 2
	buf := r.buf[r.bufIdx]
	n, err := r.r.Read(buf)
	buf = buf[:n]
	select {
	case <-r.closedCh:
	return
	case r.bufCh <- buf:
	}

However if the reader only return partial data (like it's a socket and OS chooses to return only few bytes, naming N) this reading action is finished and the loop is waiting on sending to channel (line 62). If the caller of "prefetch reader" doesn't consume the channel quickly, we are wasting a large proportion of the prefetch buffer which is buf[N:] because they are not filled. We should let the background goroutine use io.ReadFull to fully use the prefetch buffer.