Reduce coprocessor request cost

[tiancaiamao]

Enhancement

tidb/store/copr/key_ranges.go

Lines 137 to 144 in 21847fe

	ranges := make([]*coprocessor.KeyRange, 0, r.Len())
	r.Do(func(ran *kv.KeyRange) {
	ranges = append(ranges, &coprocessor.KeyRange{
	Start: ran.StartKey,
	End: ran.EndKey,
	})
	})
	return ranges

Since kv.KeyRange and coprocessor.KeyRange definition are different, we need to make object allocation to send coprocessor.Request...

We can avoid object allocation if kv.KeyRange and coprocessor.KeyRange are the same.
Actually, they are both

type KeyRange struct {
    StartKey []byte
    EndKey []byte
}