Fixes for JENKINS-22665 and JENKINS-19755

Remove the serialization of the User object within MyUserIdCause. Kept MyUserIdCause for backwards compatibility but is now just a simple subclass of the core class UserIdCause. 
This also fixes the JENKINS-19755 "Manual step "started by" username changes to anonymous after restart" since it now can load the correct user who triggered the build.
After upgrading it is possible to "Manage Old Data" to remove User object serialization of MyUserIdCause.