Asteroid/Comet mass/resources reverting to default after mining them to exhaustion

[gotmachine]

This can be easily reproduced by :

• attaching a mining setup to a brand new asteroid and using a drain valve to throw the ore away.
• engaging mining
• timewarping until asteroid depletion
• quicksaving-quickreloading
• the asteroid/comet is now back to full mass and 100% resources

The key for a consistent repro is to timewarp relatively fast, inducing a relatively large per-timestep amount of mass removed.

This seems to happen because of the very weird logic used to keep track of asteroid mass. It seems that during the last update step where there is still something to mine, if the per-timestep mined amount exceed some threshold, it leaves the data in persisted state that will be interpreted as "that asteroid part was just discovered, initialize its mass/resources" on the next load.