RANGER-4818: Fix updated users with role assignments from undergoing role reset to ROLE_USER

[kumaab]

What changes were proposed in this pull request?

Following fixes are included in this change:

• role reset for users to happen only in the last page.
• intermediate page failures (REST failure) will result in usersync retry after sync interval.
• added more context for debug messages in usersync.
• updateUsersRoles to return String instead of List<String>.

How was this patch tested?

Manually tested in docker containers (usersync and ranger-admin)