Prevent duplicate entity version numbers

[matthew-white]

Right now, it may be possible for concurrent entity updates to result in two entity versions with the same version number:

• Create a new entity. Its version number is 1.
• Send a request to update the entity. This results in a new version with version number 2.
• At the same time, send another request to update the same entity. This also results in a new version, also with version number 2.

Entity updates need to be sequential, not concurrent, because each update is applied on top of the latest entity version. Application code also assumes that the version number uniquely identifies each version of a particular entity. If there are concurrent updates, only one should succeed.

To be clear, I haven't tried to reproduce the scenario above. (@sadiqkhoja, have you?) However, I think it's important that we take steps to prevent it.