Time precision for hamlib updates

[bjorgan]

Doppler shift correction updates (and rotctld updates, but this is also limited by other factors, see #69) are currently limited to the update frequency of SingleTrack() (2 updates per second?), since these are done in tune using the same information.

Sending doppler shift corrections far more frequently than the screen update could be a nice feature for finer frequency control.

Should be done after fixing #61, with this kept in mind.