Add route back and SNR lists to RouteDiscovery for traceroute

[GUVWAF]

In order to get two-way information with SNR per hop in a traceroute, this adds another list of nodenums route_back and two lists of 1-byte SNR values (in dB, but scaled by 4, so e.g. a value of -64 is -16dB), namely snr_towards and snr_back.

This should allow us to visualize a traceroute from node 0x10 to 0x13 as:

0x10 --> 0x11 (-5.00dB) --> 0x12 (-4.00dB) --> 0x13 (7.00dB)
(-5.00dB) 0x10 <-- (-4.00dB) 0x11 <-- (7.00dB) 0x12 <-- 0x13

[thebentern]

Nice, so this should be backward compatible. Perhaps in 3.0, we can make it a repeated of structured messages rather than separate arrays, but this is a good way to ensure that things still work with older firmware versions