Avoid calling the full update loop when only validator description has changed

[cwgoes]

On handleMsgEditValidator, we presently run stakeKeeper.UpdateValidator, which runs the full power update logic - as far as I can tell, this is unnecessary, since only the description has changed. All we need to do is write the primary record in the store.

cc @rigelrozanski

[rigelrozanski]

Good catch - easy implementation