auth: get rid of deadlocking channel passing scheme in simpleTokenTTL
auth: get rid of deadlocking channel passing scheme in simpleTokenTTL

Just use the mutex instead.

Fixes #7471

mkumatag/etcd,rnaveiras/etcd,li-ang/etcd,dannysauer/etcd,bdarnell/etcd,etcd-io/etcd,mqliang/etcd,coreos/etcd,etcd-io/etcd,mkumatag/etcd,ajityagaty/etcd,handsomesun/etcd,ajityagaty/etcd,sinsharat/etcd,lorneli/etcd,gyuho/etcd,heyitsanthony/etcd,ajityagaty/etcd,joshix/etcd,lumjjb/etcd,xiang90/etcd,tguangch/etcd,YuleiXiao/etcd,etcd-io/etcd,cockroachdb/etcd,xiang90/etcd,mickep76/etcd,sinsharat/etcd,plasticbox/etcd,cockroachdb/etcd,mkumatag/etcd,glevand/coreos--etcd,joshix/etcd,jlamillan/etcd,heyitsanthony/etcd,glevand/coreos--etcd,li-ang/etcd,bluebreezecf/etcd,yudai/etcd,visheshnp/etcd,vimalk78/etcd,lorneli/etcd,sebrandon1/etcd,rnaveiras/etcd,lumjjb/etcd,armstrongli/etcd,sebrandon1/etcd,rnaveiras/etcd,mqliang/etcd,akauppi/etcd,handsomesun/etcd,gyuho/etcd,fanminshi/etcd,YuleiXiao/etcd,akauppi/etcd,whuwxl/etcd,yudai/etcd,bdarnell/etcd,mickep76/etcd,a-robinson/etcd,a-robinson/etcd,vimalk78/etcd,whuwxl/etcd,bluebreezecf/etcd,dannysauer/etcd,fanminshi/etcd,tguangch/etcd,jlamillan/etcd,armstrongli/etcd,mqliang/etcd,plasticbox/etcd,coreos/etcd,visheshnp/etcd