RegistrationLimitConstraint may fail to validate concurrent requests

Problem/Motivation

It's possible for duplicate event registrations to occur if concurrent requests are sent to the system. If multiple requests to create or update an event registration for the same event are processed in parallel, the validation may not accurately check the current status of registrations for a particular user or event.

Proposed resolution

Force the validation to acquire a lock before checking the users registration status so only one request can be processed at a time.