Extracted SQL header field expression as a function
Modified all specs to include mandatory _header return field for all Entity#get operations
Entity#get returns mandatory _header item and resolves entities into (_id, _header) pairs (albeit calculation of _header is now wrong)
Propagate engine link into entity
Added some specs for proper resolving of custom header entities on Entity#get
Propagate engine properly in all cases
Proper _header querying when doing relation resolution on Entity#get operations