NewLocalOracle creates an Oracle that uses local time as data source.
NewPdOracle create an Oracle that uses a pd client source. Refer https://github.com/tikv/pd/blob/master/client/client.go for more details. PdOracle mantains `lastTS` to store the last timestamp got from PD server. If `GetTimestamp()` is not called after `updateInterval`, it will be called by itself to keep up with the timestamp on PD server.