Synchronize access to ImpersonatedUserMetadata in OperationContext.

There is a race condition between "$currentOp" and request threads for the ImpersonatedUserMetadata in OperationContext. To prevent this, ImpersonatedUserMetadata needs to be guarded with synchronization.

ImpersonatedUserMetadata is a decoration on the OperationContext. It read/written to by the thread that owns the thread. It is read by the $currentOp thread while the $currentOp thread owns the client lock. Access to it could be guarded with the Client lock or a dedicated mutex.