INTEXT-73 Sending multipart SMS messages through a SMPP provider using sar_msg_ref_num, sar_segment_seqnum and sar_total_segments

This replaces PR https://github.com/spring-projects/spring-integration-extensions/pull/50
Since you provide a new logic for long messages to send them as `shortMessageParts` in the loop, there is just enough to remove this `Assert`, not comment