Remove user_by_token(). Split user definition and cached user object.

A few renames.
Roles (partial implementation).

Track granted roles.
Represent roles granted to a user with a bitmap.
Disallow loops in role grants.
Add tests for box.schema.user.grant('username', 'rolename')
Add basic tests for loops in grant sequences.

Rewrite largely buggy code in authentication token lookup.
Do not waste space on authentication token bitmap, correctly
check for overflow of the bitmap.
Merge branch 'master' of github.com:tarantool/tarantool