Changes

mgw: MDCX: Split mgcp header pars parsing into a previous step

Do most of the initial parsing and verification in a prior step, filling
in a "parsed" struct which simplifies logic coming after for different
message types.
This commit only modifies stuff enough to work for MDCX.
Further work (commits) will follow for DLCX.

Change-Id: I6ecb41fc2cc737c3a161b6bc98bd08ae01909655