gossip: replace scalar hops count with a slice of node IDs tracing path to origin

[spencerkimball]

Currently, gossip keeps track of the distance from a node to the origin of a piece of gossiped info via a scalar info.Hops int count which is incremented upon receipt. Various pathological restart cases result in this value being incremented arbitrarily high, giving an incorrect distant from destination node to info origin node and causing gossip connection thrashing (see #9819).

We should replace info.Hops int with info.Nodes []roachpb.NodeID and on receipt, appropriately collapse the supplied slice of node IDs to the shortest sub-slice which contains an immediate peer node ID.

For example:

Node(1..N) = N1, N2, N3, ...
Node(origin) = N1
Node(dest) = N4

If No receives info.Nodes = []roachpb.NodeID{N1, N2, N3, N5, N6}, and No has peers []roachpb.NodeID{N2, N7, N9}, it will set info.Nodes = []roachpb.NodeID{N1, N2}.