more correct creation and edit times for entities

Fixing some bugs that I introduced when I split out EntityItem::_lastSimulated time from EntityItem::_lastUpdated and EntityItem::_lastEdited.

The bugs were not noticeable under normal conditions, but became noticeable when trying to get results from one physics simulation (bullet) to be relayed across the EntityServer and into another interface instance that was running a simpler simulation.  The update packets from the EntityServer were getting rejected at the receiving end because the accompanying _lastEdited time was too old.
I think that by removing the hasGravity() from this, you will break the case where an entity has no initial velocity, but has some gravity... it seems like in this case you'll never enter this if branch, and you will never apply the gravity to give it it's first initial velocity.