Feature Request: Support deliver_sm_resp PDUs

[amalg]

Currently, deliver_sm PDUs arrive with registered_delivery bits are all set to 0, meaning no deliver_sm_resp is required from the ESME. While it is not necessary to change this to request deliver_sm_resp PDUs, support for response PDUs would be nice.

I currently receive and store messages in a database, and when the database is reachable I send back an ESME_ROK response PDU. However, should the database be inaccessible, I would like to be able to send back an ESME_RDELIVERYFAILURE response PDU and have shorty retry message delivery for up to 60 minutes (1 hour).