variable: fix `NewSessionVars` (#6272)
variable: fix `NewSessionVars` (#6272)

Set IndexLookupJoinConcurrency to default value.
If this value is init as 0, index lookup join will hang forever.
pingcap/tidb,ngaut/tidb,XuHuaiyu/tidb,wusphinx/tidb,kyle-wang/tidb,kyle-wang/tidb,ngaut/tidb,Nify/tidb,tiancaiamao/tidb,ngaut/tidb,pingcap/tidb,Nify/tidb,kyle-wang/tidb,pingcap/tidb,ngaut/tidb,c4pt0r/tidb,CodeRushing/tidb,ngaut/tidb,wusphinx/tidb,Cofyc/tidb,qiuyesuifeng/tidb,c4pt0r/tidb,cosmtrek/tidb,c4pt0r/tidb,coocood/tidb,kyle-wang/tidb,coocood/tidb,tiancaiamao/tidb,CodeRushing/tidb,ming-relax/tidb,EvilMcJerkface/tidb,c4pt0r/tidb,ming-relax/tidb,pingcap/tidb,c4pt0r/tidb,XuHuaiyu/tidb,pingcap/tidb,mccxj/tidb,EvilMcJerkface/tidb,qiuyesuifeng/tidb,mccxj/tidb,kyle-wang/tidb,Cofyc/tidb,kyle-wang/tidb,pingcap/tidb,c4pt0r/tidb,ngaut/tidb,cosmtrek/tidb