benchdb: bootstrap session. (#2726)
benchdb: bootstrap session. (#2726)

* benchdb: use random string for every row

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