kvencoder: fix data race on plan.PreparedPlanCacheCapacity (#6159)
kvencoder: fix data race on plan.PreparedPlanCacheCapacity (#6159)


tiancaiamao/tidb,kyle-wang/tidb,CodeRushing/tidb,coocood/tidb,coocood/tidb,EvilMcJerkface/tidb,pingcap/tidb,pingcap/tidb,c4pt0r/tidb,pingcap/tidb,pingcap/tidb,mccxj/tidb,c4pt0r/tidb,wusphinx/tidb,ngaut/tidb,Cofyc/tidb,qiuyesuifeng/tidb,kyle-wang/tidb,ngaut/tidb,Nify/tidb,kyle-wang/tidb,kyle-wang/tidb,cosmtrek/tidb,c4pt0r/tidb,c4pt0r/tidb,tiancaiamao/tidb,ngaut/tidb,mccxj/tidb,pingcap/tidb,Nify/tidb,pingcap/tidb,CodeRushing/tidb,XuHuaiyu/tidb,Cofyc/tidb,ngaut/tidb,EvilMcJerkface/tidb,ngaut/tidb,c4pt0r/tidb,kyle-wang/tidb,cosmtrek/tidb,ming-relax/tidb,XuHuaiyu/tidb,qiuyesuifeng/tidb,ming-relax/tidb,c4pt0r/tidb,ngaut/tidb,wusphinx/tidb,kyle-wang/tidb