add a step field in WS2P Head
The idea is to estimate the density of the WS2P network by knowing for each head by how many nodes it passed before reaching itself.
The node will initialize its own head with step=0 Each time the node relays headers it increments their step field.
Note that this step field will not be signed. It will provide purely indicative data and will nevertheless be useful for statistical purposes if it is assumed that the majority of the nodes in the network do not lie.