Avatars: Loading state and physics enabling

[leanmendoza]

When somebody teleports to a place where its spawn point is not on the floor, the avatar flies the all loading time.

The evidence case is Genesis plaza and the workaround at the moment is to avoid this falling and change the spawn point in this PR.

This happens because before the loading we don't know what collider could affect the falling, so physics is paused until the loading has finished. But other peers can see the avatar flying because its position isn't affected by the physics.

A more final solution could be that exists a loading tag that allows hiding the avatars with a loading state.