Use new Rrd_timescales module

Currently we're just using a list of tuples [here](https://github.com/xapi-project/xcp-rrdd/blob/master/rrdd/rrdd_shared.ml#L52-64) whereas we could be using the new [Rrd_timescales](https://github.com/xapi-project/xcp-rrd/blob/master/lib/rrd_timescales.mli) module.
