Insert a short delay on the simulator side rezzing of attachments in order to fix viewer 3 issues if its own rezzing actions on login collide with the simulator side actions.

This resolves issues (at least in my tests with LL 3.3.4) where this can make attachments invisible until one zooms in on the avatar.
This doesn't affect version 1 viewers since this delay is shorter than the login delay.
This doesn't increase the login time since this part of the process was already being performed asynchronously.
This may be a temporary solution.