P2P sync improvements

[heifner]

• Add new option sync-peer-limit to limit the number of peers to sync from. Default 3.
• Track latency of peers and only sync from sync-peer-limit with lowest latency.
• Change default sync-fetch-span from 100 to 1000.
• Add new network protocol version to include start/end range in notice_message. Only attempt to sync from peers with range of required blocks.
• Fix: do not save incoming trxs into unapplied queue while syncing.

Resolves #1072

[greg7mdp]

I was wondering why we use these 3 states:

      std::atomic<bool>       connecting{true};
      std::atomic<bool>       syncing{false};
      std::atomic<bool>       closing{false};

instead of having a single state such as:

enum class connection_state { connecting, connected, syncing, synced, closing, closed  };

Can we be in multiple states at once?

[heifner]

I was wondering why we use these 3 states:

      std::atomic<bool>       connecting{true};
      std::atomic<bool>       syncing{false};
      std::atomic<bool>       closing{false};

instead of having a single state such as:

enum class connection_state { connecting, connected, syncing, synced, closing, closed  };

Can we be in multiple states at once?

The net_plugin could do for a refactor to use a state machine. This looks like low-hanging fruit to me. I'll go ahead and clean that up.