raft: use mutex in "SetLogger" to avoid race conditions in tests
raft: use mutex in "SetLogger" to avoid race conditions in tests

Signed-off-by: Gyuho Lee <0fbd30d364d71722b926950b451cecd97b54fc29@amazon.com>

mqliang/etcd,cockroachdb/etcd,coreos/etcd,etcd-io/etcd,rnaveiras/etcd,etcd-io/etcd,ajityagaty/etcd,mkumatag/etcd,mkumatag/etcd,cockroachdb/etcd,ajityagaty/etcd,li-ang/etcd,dannysauer/etcd,rnaveiras/etcd,gyuho/etcd,mqliang/etcd,gyuho/etcd,li-ang/etcd,mkumatag/etcd,mqliang/etcd,coreos/etcd,ajityagaty/etcd,etcd-io/etcd,rnaveiras/etcd,dannysauer/etcd