[Issue 1278] Rework static peer discovery handling

[mbaxter]

PR description

Rework discovery-level handling of static peers to fix a few issues:

• Make sure that permissions are respected when pushing static peers into the discovery layer
• Initiate ping-pong bonding cycle rather than directly adding peers to our peer table to ensure that peer is live and running discovery
• Check that enode url indicates discovery protocol is running before attempting to bond with peer
• Remove duplicate dropPeer logic

Fixed Issue(s)

Fixes #1278

[lucassaldanha]

Does this mean that we require discovery to be enabled on peers that we want to add using static-nodes.json file or AddPeer API? Because I feel like most uses that are statically mapping their peers might not have discovery enabled (and might not want to).

[mbaxter]

No - we try to create a DiscoveryPeer to initiate bonding with. The DiscoveryPeer.from helper will only return a peer if discovery appears to be enabled. The bonding process consists of sending a PING to that peer, and waiting for a response. Only if the peer is running discovery and responds do we add the peer to our table. If the peer doesn't respond, we'll retry a few times and then stop.

Users can explicitly indicate that discovery is disabled by supplying enode urls with discport=0, which will cause us to skip the bonding attempt.