[net processing] Move hashContinue to net processing

Also rename to m_continuation_block to better communicate meaning.

src/net_processing.h

@@ -76,6 +76,11 @@ 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 */
+    uint256 m_continuation_block{};
 
     /** Set of txids to reconsider once their parent transactions have been accepted **/
     std::set<uint256> m_orphan_work_set GUARDED_BY(g_cs_orphans);