embed: support "io-discard" for logging, fix racey logging setup
embed: support "io-discard" for logging, fix racey logging setup

Signed-off-by: Gyuho Lee <3454051ee3341eb69bdf6fca40899cdad4d86496@gmail.com>

bdarnell/etcd,coreos/etcd,mqliang/etcd,vimalk78/etcd,sinsharat/etcd,mkumatag/etcd,li-ang/etcd,glevand/coreos--etcd,joshix/etcd,li-ang/etcd,xiang90/etcd,rnaveiras/etcd,mkumatag/etcd,ajityagaty/etcd,etcd-io/etcd,dannysauer/etcd,cockroachdb/etcd,ajityagaty/etcd,mqliang/etcd,fanminshi/etcd,bdarnell/etcd,coreos/etcd,mkumatag/etcd,lorneli/etcd,sebrandon1/etcd,xiang90/etcd,etcd-io/etcd,lorneli/etcd,dannysauer/etcd,etcd-io/etcd,glevand/coreos--etcd,vimalk78/etcd,joshix/etcd,ajityagaty/etcd,sinsharat/etcd,gyuho/etcd,gyuho/etcd,fanminshi/etcd,rnaveiras/etcd,sebrandon1/etcd,mqliang/etcd,rnaveiras/etcd,cockroachdb/etcd