Reason: trying to make unique string for every possible argument in
global fashion is difficult. For example it would make sense for
EditPostRelationsJob to accept argument named "post-ids", but it
wouldn't make much sense for AddPostJob to accept "post-ids" since it
doesn't tell much. Thus, common arguments are going to be defined in
top-level AbstractJob for ease of control, while more job-specific
arguments are going to be specified in respective job implementations.
