config: limit backup workers cap (#12141)
config: limit backup workers cap (#12141)

ref #11804

Signed-off-by: Wenbo Zhang <a95f50f76a84613da6546144d6ff1b2dd9847c90@gmail.com>

Co-authored-by: Ti Chi Robot <040e6455cde1c91e876ec337e785ce913eacba35@tidb.io>
BusyJay/tikv,BusyJay/tikv,BusyJay/tikv,BusyJay/tikv