Add uniqueness check to Service#queue() if Service(unique=True)
Change a log message to debug level in Service._worker()
Add the requirement key to the Maestro.requirement_structure
Simplify the Maestro.requirement_structure() and clean some dead code
Fix Curdler.handler() removing the requirement from parameter list
Both `requirement` and `wheel` are required parameters for the Dependencer
More data for debugging exceptions in Service._worker()