[net processing] Move hashContinue to net processing

Summary:
```
Also rename to m_continuation_block to better communicate meaning.
```

Partial backport of [[bitcoin/bitcoin#19829 | core#19829]]:
bitcoin/bitcoin@184557e

Depends on D10870.

Ref T1696.

Test Plan:
  ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1696

Differential Revision: https://reviews.bitcoinabc.org/D10871

Author: jnewbery

src/net.cpp

@@ -3335,7 +3335,6 @@ CNode::CNode(NodeId idIn, ServiceFlags nLocalServicesIn,
       m_inbound_onion(inbound_onion) {
     hSocket = hSocketIn;
     addrName = addrNameIn == "" ? addr.ToStringIPPort() : addrNameIn;
-    hashContinue = BlockHash();
     if (conn_type_in != ConnectionType::BLOCK_RELAY) {
         m_tx_relay = std::make_unique<TxRelay>();
     }

src/net.h

@@ -1096,8 +1096,6 @@ class CNode {
     mapMsgCmdSize mapRecvBytesPerMsgCmd GUARDED_BY(cs_vRecv);
 
 public:
-    BlockHash hashContinue;
-
     // flood relay
     std::vector<CAddress> vAddrToSend;
     std::unique_ptr<CRollingBloomFilter> m_addr_known = nullptr;

src/net_processing.cpp

@@ -1894,7 +1894,7 @@ static void RelayAddress(const CNode &originator, const CAddress &addr,
     connman.ForEachNodeThen(std::move(sortfunc), std::move(pushfunc));
 }
 
-static void ProcessGetBlockData(const Config &config, CNode &pfrom,
+static void ProcessGetBlockData(const Config &config, CNode &pfrom, Peer &peer,
                                 const CInv &inv, CConnman &connman,
                                 const std::atomic<bool> &interruptMsgProc) {
     const Consensus::Params &consensusParams =
@@ -2057,15 +2057,15 @@ static void ProcessGetBlockData(const Config &config, CNode &pfrom,
 
         // Trigger the peer node to send a getblocks request for the next batch
         // of inventory.
-        if (hash == pfrom.hashContinue) {
+        if (hash == peer.m_continuation_block) {
             // Send immediately. This must send even if redundant, and
             // we want it right after the last block so they don't wait for
             // other stuff first.
             std::vector<CInv> vInv;
             vInv.push_back(
                 CInv(MSG_BLOCK, ::ChainActive().Tip()->GetBlockHash()));
             connman.PushMessage(&pfrom, msgMaker.Make(NetMsgType::INV, vInv));
-            pfrom.hashContinue = BlockHash();
+            peer.m_continuation_block = BlockHash();
         }
     }
 }
@@ -2257,7 +2257,8 @@ static void ProcessGetData(const Config &config, CNode &pfrom, Peer &peer,
     if (it != peer.m_getdata_requests.end() && !pfrom.fPauseSend) {
         const CInv &inv = *it++;
         if (inv.IsGenBlkMsg()) {
-            ProcessGetBlockData(config, pfrom, inv, connman, interruptMsgProc);
+            ProcessGetBlockData(config, pfrom, peer, inv, connman,
+                                interruptMsgProc);
         }
         // else: If the first item on the queue is an unknown type, we erase it
         // and continue processing the queue on the next call.
@@ -3531,7 +3532,7 @@ void PeerManager::ProcessMessage(const Config &config, CNode &pfrom,
                 // trigger the peer to getblocks the next batch of inventory.
                 LogPrint(BCLog::NET, "  getblocks stopping at limit %d %s\n",
                          pindex->nHeight, pindex->GetBlockHash().ToString());
-                pfrom.hashContinue = pindex->GetBlockHash();
+                peer->m_continuation_block = pindex->GetBlockHash();
                 break;
             }
         }

src/net_processing.h

@@ -93,6 +93,13 @@ struct Peer {
 
     /** This peer's reported block height when we connected */
     std::atomic<int> m_starting_height{-1};
+    /**
+     * The final block hash that we sent in an `inv` message to this peer.
+     * When the peer requests this block, we send an `inv` message to trigger
+     * the peer to request the next sequence of block hashes.
+     * Most peers use headers-first syncing, which doesn't use this mechanism
+     */
+    BlockHash m_continuation_block{};
 
     /**
      * Set of txids to reconsider once their parent transactions have been