Estimate network size as per @Thierry61's suggestion

[michaelsproul]

Updated version of #1353

[maidsafe-highfive]

r? @afck

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[michaelsproul]

Here's what it looks like in the perfect and imperfect information cases respectively:

INFO 07:02:49.244218000 [routing::states::node node.rs:281]  ---------------------------------------------------------- 
INFO 07:02:49.244270000 [routing::states::node node.rs:282] | Node(55a5f3..(010)) PeerId(19) - Routing Table size:  43 |
INFO 07:02:49.244296000 [routing::states::node node.rs:283] | Exact network size: 44                                   |
INFO 07:02:49.244328000 [routing::states::node node.rs:284]  ---------------------------------------------------------- 

INFO 07:02:48.592343000 [routing::states::node node.rs:281]  ---------------------------------------------------------- 
INFO 07:02:48.592373000 [routing::states::node node.rs:282] | Node(223bf6..(001)) PeerId(11) - Routing Table size:  36 |
INFO 07:02:48.592397000 [routing::states::node node.rs:283] | Estimated network size: 43                               |
INFO 07:02:48.592428000 [routing::states::node node.rs:284]  ----------------------------------------------------------