Fixes #659

This PR adds startup options (available via both command line and the config.ini configuration file) to witness_node:

1. p2p-accept-incoming-connections will allow peers to request a connection to your node (default is true). Set to false, your node will not listen for incoming connections. The "accept-incoming-connections" is an existing field in the node configuration file (p2p/node_config.json), now accessible from the command line and the config.ini configuration file.
2. p2p-inbound-endpoint, used to specify the node's "external" IP address and listening port when it is behind DNAT or a reverse proxy.
3. p2p-connect-to-new-peers will allow the node to connect to new peers advertised by other peers (default is true). Set to false, the node will ignore all peer advertisements.
4. p2p-advertise-peer-algorithm determines how peers are selected to be advertised.
5. p2p-advertise-peer-endpoint and p2p-exclude-peer-endpoint work in conjunction with some of the peer algorithms.

The peer algorithms that can be used are:

1. nothing which will respond to the requesting peer with an empty list
2. list which will respond with a list of connected peers which are also in the list provided by p2p-advertise-peer-endpoint
3. exclude_list which will respond with a list of connected peers which are not in the list provided by p2p-exclude-peer-endpoint
4. all, or any other value, or if no peer algorithm is provided, all connected peers are advertised as they were before this enhancement.

Other changes and improvements:

• Node will listen to the default port (1776) by default, rather than a random port. If p2p-endpoint option is not specified, when the default port is unavailable, the node will listen on a random port.
• Node no longer performs firewall check requested by other peers, nor ask other peers to perform firewall check for itself or another peer. The node will perform firewall check / detection on its own on incoming peers.
• Node will only accept an address_message if it has just requested one
• Node only handles at most 200 addresses for each address_message
• Added some gate-keeping code, to disconnect or ignore when received unexpected messages
• Node will advertise other peers' inbound endpoints but not necessarily the remote endpoints of the current connections. If a peer's inbound port is 0, its address will still be advertised.
• If node is not connected to any peer, it rechecks its seed node list every 5 minutes; otherwise it rechecks the seed node list every 1 hour (was always recheck every 3 hours)
• Node will try to guess and verify the inbound peers' inbound (listening) endpoints
• Peer's number_of_failed_connection_attempts will be halved on successful outbound connection
• Refactored & optimized the network_mapper program
• Added fc::ip::address::is_loopback_address() (127.*.*.*), fixed is_public_address() to detect loopback addresses