Cleanup Veritas::Adapter::Arango::Visitor::Header

Nice metrics impact!
Whitespace
Cleanup wrap or not to wrap in FOR node decision

* Use the inheritance tree for decision
* Abstract method :leaf can be declared on the shared superclass
* Move Reverse visitor out of that namespace
* Cleanup older specs with improvements of expect_aql macro
Deduplicate #local_name implementations in Wrappable namespace