Break configured_limits to their own file
Stub parting optargs in.
Add limits to read_t and write_t.

Tim suggested this, and it seems to be productiev although protocol is failing with a weird error message.
Add copy constructor, since apparently we need it.
These do actually need to be modifiable.
Remove explicit-only copy constructor
We're initializing using an empty global optargs list, so use default
Fix otherwise inexplicable compile errors by making this field nonconst
Propogate read_t & write_t constructor changes