Clear caches if race condition detected in CentralAuthUser::attach

Change-Id: Iaefaa69966328cf1246d2bea30cbe9b1d2d842d9
(cherry picked from commit c3295b662aa8f01a2947a94b810653ef6072d411)
Merge "Clear caches if race condition detected in CentralAuthUser::attach" into wmf/1.24wmf15