storage/reader: Avoid some copy in mvcc reader (#4373)
storage/reader: Avoid some copy in mvcc reader (#4373)

* Avoid some copy in mvcc reader

Signed-off-by: MyonKeminta <f61fcbf0b1b8e66e29bfd79a313ac22303163882@users.noreply.github.com>
c4pt0r/tikv,pingcap/tikv,BusyJay/tikv,pingcap/tikv,BusyJay/tikv,BusyJay/tikv,BusyJay/tikv