import awesome_nested_set 2.1.6 (#6579)

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

--HG--
extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/trunk%4013007
use bundled awesome_nested_set (#6579)

--HG--
extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/trunk%4013008
fix always new lft and rgt are lft = 1, rgt = 2 (#6579)

--HG--
extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/trunk%4013009
awesome_nested_set: split highest rgt reader method (#6579)

--HG--
extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/trunk%4013010
fix race condition of highest rgt at Issue#update_nested_set_attributes_on_parent_change (#6579)

--HG--
extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/trunk%4013011