Change capped collection age-out to be based on collection storageSize, not dataSize

Due to SERVER-35431, the dataSize reported for a collection can be incorrect in 4.0, even without unclean crashes. We should switch capped collection sizes to be based on the storageSize, not the dataSize, so that the calculations can be accurate. We also believe this is more likely to be what users actually want when they configure a capped collection anyway, as they are generally trying to limit the disk space consumed by the collection.