ddl: Speed up add index phase (#2260)
ddl: Speed up add index phase (#2260)

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