optimizer/plan: rename index to offset
optimizer/plan: rename index to offset

tiancaiamao/tidb,kyle-wang/tidb,gaocegege/tidb,louishust/tidb,silenceper/tidb,yangwenmai/tidb,ming-relax/tidb,silentred/tidb,zs634134578/tidb,louishust/tidb,zimulala/tidb,mccxj/tidb,coocood/tidb,ueizhou/tidb,cosmtrek/tidb,wusphinx/tidb,liyu4/tidb,ngaut/tidb,kyle-wang/tidb,c4pt0r/tidb,pingcap/tidb,andelf/tidb,spongedu/tidb,c4pt0r/tidb,wusphinx/tidb,liyu4/tidb,cosmtrek/tidb,qhsong/tidb,c4pt0r/tidb,ngaut/tidb,JinheLin/tidb,qiuyesuifeng/tidb,pingcap/tidb,pingcap/tidb,Cofyc/tidb,EvilMcJerkface/tidb,astaxie/tidb,Cofyc/tidb,kyle-wang/tidb,mccxj/tidb,ngaut/tidb,c4pt0r/tidb,8cbx/tidb,yuanwhy/tidb,simon-xia/tidb,c4pt0r/tidb,Nify/tidb,EvilMcJerkface/tidb,JinheLin/tidb,zs634134578/tidb,CodeRushing/tidb,pingcap/tidb,tiancaiamao/tidb,anywhy/tidb,ueizhou/tidb,myrice/tidb,kyle-wang/tidb,Nify/tidb,zimulala/tidb,silentred/tidb,gaocegege/tidb,XuHuaiyu/tidb,kyle-wang/tidb,simon-xia/tidb,CodeRushing/tidb,8cbx/tidb,XuHuaiyu/tidb,myrice/tidb,silenceper/tidb,qhsong/tidb,pingcap/tidb,kyle-wang/tidb,coocood/tidb,spongedu/tidb,anywhy/tidb,andelf/tidb,ngaut/tidb,yuanwhy/tidb,yangwenmai/tidb,ngaut/tidb,ming-relax/tidb,qiuyesuifeng/tidb,ngaut/tidb,astaxie/tidb,c4pt0r/tidb,pingcap/tidb