Investigate eliminating extra component removal messages in sync code

Right now when deleting entities component removal events are generated for tracked components. If more entities are generated in the same tick, there is no way to differentiate where the removal events are from because the don't include the entity generation number forcing us to send extra events to the client that are unnecessary since the entity was already removed.

We might even do this without entities being created but that is more easily fixable.

• Investigate if we send removed events for entities that are no longer around and fix if so by checking if the entity still exists
• Investigate how we can fix the issue of determining where the removal event is from when multiple entities are created/deleted at the same index in a single tick (specs issue: https://github.com/amethyst/specs/issues/720)