Mica's USSD over SMPP implementation details have changed

A bit cryptic but this:

```
1. user_message_reference is used as the identifier with (and only with!) source
number. to identify session with timeout based identification. 
user_message_reference type is Short -- it can't be used as a global identifier. 

2. If you need global identifier, you can implement global session as  source +
user_msg_ref internally on your side (as per docs) and it will identify session. 
if somehow old session not cleared, value of user_msg_ref will be incremented by 
ussdc until clearing time of obsolete sessions. 

3. destination_addr contains shortCode used, short_message contains actual 
ussdString from the network (handset).
```