Use esm_class field in SMPP PDUs to check for delivery reports

According to section 5.2.12 of the SMPP 3.4 spec, we can use the middle four bits of the `esm_class` field to determine the message type and improve our delivery report detection accuracy.
I'd rather not add this complexity unless we actually need it -- so far all the SMSCs we've received DRs from recently have had the relevant `esm_class` flags set. Considering that the DR content format is actually a valid message, I've always been somewhat uncomfortable using a regex match to detect them. (Especially since the regex has failed for us a number of times already.)