Improve P2P DoS resilience

[xgreenx]

Currently, the external sentries in the network are very vulnerable to external attackers. The attacker can spam nodes with many connections, requests, and heavy data-consuming sentry resources.

It can halt the node or cause resource exhaustion or OOM. Following issue highlights part of the vectors of atackes:

• Add config options for max pending connections, for both outgoing and incoming #904
• Peer Reputation for syncing #943
• Close bad connection as notified by Heartbeat in P2P code #1567
• Add a libp2p rate limiting behavior #1677
• Bootstrap nodes should help discover other nodes in the network before shutting down the connections #1774 - Without it attacker can consume all available connections