fix tso bench cannot exit (#3217)
fix tso bench cannot exit (#3217)

Signed-off-by: Ryan Leung <rleungx@gmail.com>

Co-authored-by: Ti Prow Robot <8d93ecb880184f72dbfca8f10b11aa46074d2fca@users.noreply.github.com>
pingcap/pd,pingcap/pd