Periodically make block-relay connections and sync headers

To make eclipse attacks more difficult, regularly initiate outbound connections
and stay connected long enough to sync headers and potentially learn of new
blocks. If we learn a new block, rotate out an existing block-relay peer in
favor of the new peer.

This augments the existing outbound peer rotation that exists -- currently we
make new full-relay connections when our tip is stale, which we disconnect
after waiting a small time to see if we learn a new block.  As block-relay
connections use minimal bandwidth, we can make these connections regularly and
not just when our tip is stale.

Like feeler connections, these connections are not aggressive; whenever our
timer fires (once every 5 minutes on average), we'll try to initiate a new
block-relay connection as described, but if we fail to connect we just wait for
our timer to fire again before repeating with a new peer.

src/net_processing.h

@@ -206,6 +206,10 @@ private:
     //* Whether this node is running in blocks only mode */
     const bool m_ignore_incoming_txs;
 
+    /** Whether we've completed initial sync yet, for determining when to turn
+      * on extra block-relay-only peers. */
+    bool m_initial_sync_finished{false};
+
     /** Protects m_peer_map */
     mutable Mutex m_peer_mutex;
     /**