+ Returns a thread safe monotonically increasing id suitable for use
+ as a globally unique identifier.
+
+ >>> import id_generator
+ >>> id_generator.get('student_id')
+ 0
+ >>> id_generator.get('student_id')
+ 1
+ >>> id_generator.get('employee_id', start=10000)
+ 10000
+ >>> id_generator.get('employee_id', start=10000)
+ 10001