Polish coordinator/provider/acomm

[bbickerton]

Review function signatures and flows after a real provider (e.g. #40 ) has used them. Find anything that doesn't work well or feels clunky and address before making more providers.

[mmlb]

adding "" for arg in NewRequest is particularly wrong looking. Maybe NewRequest should just take in the args it needs for validation and returns a default+validated-args Request instead of filling it all in.