pkg/types: Avoid potential double lock of tsafeSet.
pkg/types: Avoid potential double lock of tsafeSet.

(tsafeSet).Sub and (tsafeSet).Equals can cause double lock bug if ts and other is pointing the same variable

gofmt the code and add some comments

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