PacketID is bloated. Some minor reordering, which shouldn't effect
locality, reduces sizeof(PacketID) from 232 to 208 bytes compiled
against GNU libstdc++ (shipped with GCC 5.3, C++11 ABI) and from 184 to
160 bytes against libc++ 3.7.0. In both cases only 3 bytes of padding
remain for aligning 'bool inIncompleteOkay'.