Merge #19858: Periodically make block-relay connections and sync headers

b3a515c0be Clarify comments around outbound peer eviction (Suhas Daftuar)
daffaf03fb Periodically make block-relay connections and sync headers (Suhas Daftuar)
3cc8a7a0f5 Use conn_type to identify block-relay peers, rather than m_tx_relay == nullptr (Suhas Daftuar)
91d61952a8 Simplify and clarify extra outbound peer counting (Suhas Daftuar)

Pull request description:

  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.

ACKs for top commit:
  ariard:
    Code Review ACK b3a515c, only change since last time is dropping a useless `cs_main` taking. I manually tested a previous version of the PR, and not substantial change has been introduced since then which would alter behavior IMO.
  jonatack:
    Tested ACK b3a515c0be over several weeks, though this change and behavior could benefit from test coverage and other follow-ups (refactoring, etc.) described in the review feedback. I did not verify the behavior of `m_start_extra_block_relay_peers` only being enabled after initial chain sync. Since my last review, one unneeded `cs_main` lock was removed.

Tree-SHA512: 75fc6f8e8003e88e93f86b845caf2d30b8b9c0dbb0a6b8aabe4e24ea4f6327351f736a068a3b2720a8a581b789942a3a47f921e2afdb47e88bc50d078aa37b6f

src/test/fuzz/connman.cpp

@@ -145,7 +145,7 @@ void test_one_input(const std::vector<uint8_t>& buffer)
     }
     (void)connman.GetAddedNodeInfo();
     (void)connman.GetBestHeight();
-    (void)connman.GetExtraOutboundCount();
+    (void)connman.GetExtraFullOutboundCount();
     (void)connman.GetLocalServices();
     (void)connman.GetMaxOutboundTarget();
     (void)connman.GetMaxOutboundTimeframe();