rpc: warn that nodes ignore requests for old stale blocks

This is an anti-fingerprinting measure. See BlockRequestAllowed in net_processing.

It has been around since 2014, but alternative clients might still serve these blocks.

See also: d8b4b49667, 85da07a5a0, a2be3b66b5, 3788a8479b

src/net_processing.cpp

@@ -1738,6 +1738,8 @@ std::optional<std::string> PeerManagerImpl::FetchBlock(NodeId peer_id, const CBl
     LOCK(cs_main);
 
     // Mark block as in-flight unless it already is (for this peer).
+    // If the peer does not send us a block, vBlocksInFlight remains non-empty,
+    // causing us to timeout and disconnect.
     // If a block was already in-flight for a different peer, its BLOCKTXN
     // response will be dropped.
     if (!BlockRequested(peer_id, block_index)) return "Already requested from this peer";