This is mostly a hack to allow IPA/SCCP routing to SCTP/M2UA/MTP3/SCCP
without going through the full stack. The proper way of doing this
requires another round of abstraction for the mtp_link_set class.
For the ISUP/MGCP handling we will need the same code, extract it
from the msc_connection. For the reading code callback is introduced
that will pass the MGCP message to the higher layer.