import awesome_nested_set 2.1.6 (#6579)

https://github.com/collectiveidea/awesome_nested_set/commit/64cc8bc8cf93e8859a21bcf25c36675080e42ce6

git-svn-id: http://svn.redmine.org/redmine/trunk@13007 e93f8b46-1217-0410-a6f0-8f06a7374b81
use bundled awesome_nested_set (#6579)

git-svn-id: http://svn.redmine.org/redmine/trunk@13008 e93f8b46-1217-0410-a6f0-8f06a7374b81
fix always new lft and rgt are lft = 1, rgt = 2 (#6579)

git-svn-id: http://svn.redmine.org/redmine/trunk@13009 e93f8b46-1217-0410-a6f0-8f06a7374b81
awesome_nested_set: split highest rgt reader method (#6579)

git-svn-id: http://svn.redmine.org/redmine/trunk@13010 e93f8b46-1217-0410-a6f0-8f06a7374b81
fix race condition of highest rgt at Issue#update_nested_set_attributes_on_parent_change (#6579)

git-svn-id: http://svn.redmine.org/redmine/trunk@13011 e93f8b46-1217-0410-a6f0-8f06a7374b81