Add / Handle OAuth2 & Replicas incompatibility

The OAuth2 user sync is currently incompatible with Gentics Mesh replicas. It is not possible to determine (e.g. in Load balancer) whether a request will cause a user sync or not.

Possible solutions / workarounds:

* Throw error when a read request on a replica causes a oauth user sync
* Redirect user sync to master instance (May not be possible due to deferred transaction sync)
* Don't sync user - Instead just use a shallow user within the routing context which is not persisted (May not work since roles / groups would need to be synced as well)