smarter determination of the transport client

[kangguru]

Right now the transport_client is either determined by having signature_class == "T" (where signature_class will never be set on the subscriber anyways) or the first subscriber on the account. This raises a Problem when this subscriber isn't allowed to perform the needed operation namely HAC or STA

Also adding a new subscriber will keep the system broken, as still the first subscriber is selected.

Proposal: Store accessible order types by subscriber and use the info to select the correct client. Also helps/comes with #147

[kangguru]

preparing work is already in the making railslove/epics#57