Tame metadata in packet structure

[daniel-noland]

The size of this struct is way out of control

Minimum requirement should be 2x 64B cache lines. 3 is silly. 1 is ideal, but that may be impractical.

[qmonnet]

Field bridge (8 bytes) is currently unused

[EDIT]: Fredi asks to keep it

[qmonnet]

Also todo: test or static assert to ensure the size remains under some limit