network/kademlia: Prioritize closest peer when suggesting peer for neighborhood

[aperonam]

As described in https://github.com/nolash/SWIPs/blob/hive/SWIPs/swip-hive.md, when suggesting peers, if unconected peers in the neighborhood, it should always prioritize connecting to the closest neighbor, e.g

=========================================================================
population: 2 (4), NeighbourhoodSize: 2, MinBinSize: 2, MaxBinSize: 16
============ DEPTH: 0 ==========================================
000  0                              |  1 f800 (0)
001  0                              |  1 7000 (0)
002  1 2000                       |  1 2000 (0)
003  1 1000                       |  1 1000 (0)
004  0                              |  0
005  0                              |  0
006  0                              |  0
007  0                              |  0
008  0                              |  0
009  0                              |  0
010  0                              |  0
011  0                              |  0
012  0                              |  0
013  0                              |  0
014  0                              |  0
015  0                              |  0
========================================================================= 

In this situation , although bin0 is shallower, since both bin1 and bin0 are in the neighborhood, the suggested peer should be for bin1 (closest neighbor)

This PR modifies kademlia SuggestPeers method in order to prioritize connecting to the closest neighbor.
TestSuggestPeer has also been modified to explicitly test this behavior

Fixes #1897

[nolash]

currBinIndex = po?

[nolash]

Won't this mean that you continue processing all bins up to 0 in the order nearest to farthest? The ordering should only be up to depth. Shallower than depth the existing algorithm should be used.