tidb-server: support schema lease flag.
tidb-server: support schema lease flag.

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