refactor(kademlia): speeding up topology build

[mrekucci]

Speeds up the topology build by dividing the connect neighbor
method into two separate phases executed in sequential order.
The first phase makes a number of concurrent connections not
bigger than what is considered for the bin to be saturated. The
second phase takes care of the rest of the connection in a
regular manner.

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This change is 

[Eknir]

I got some data on the speed of Kademlia build up in the branch versus master

Master (11m total):
922: 0
925: depth 1
927: depth 3
931: depth 5
934: depth 15

This branch (3m total):
912: 0
913: depth 3
915: depth 7
916: depth 15

I collected these metrics manually (refreshing page in browser). Would be very nice if we can have metrics for this, so we can also do more proper benchmarking.

[Eknir]

approving based on the fact that this speeds up Kademlia build up with about 3x

[acud]

Reviewed 3 of 3 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @metacertain)