REFACTOR: don't pass built maps between processes

[ColmTalbot]

This PR makes two changes:

• refactor how the coadding is implemented so that there are two stages (identifying which maps should be coadded using metadata, actually doing the coadding) so that built maps don't need to be passed between processes. This seems to be reducing our memory footprint some.
• don't pass the maps to the output classes. We discussed this on a previous call and there is only a small loss of information. The lightcurvedb upload is now missing the time, I assume we want to preserve that, but I'm not immediately sure how to pass it.

[JBorrow]

Seems sensible to me.

[axf295]

Is there a config for the coadding so that coadding can be turned on or off?

[axf295]

seems like the lightcurvedb should expect the source observation time to come from the measured source object, not require map_time @JBorrow