wallet2: fix rescanning tx via scan_tx

- Detach & re-process txs >= lowest scan height
- ensures that if a user calls scan_tx(tx1) after scanning tx2,
the wallet correctly processes tx1 and tx2
- if a user provides a tx with a height higher than the wallet's
last scanned height, the wallet will scan starting from that tx's
height
- scan_tx requires trusted daemon iff need to re-process existing
txs: in addition to querying a daemon for txids, if a user
provides a txid of a tx with height *lower* than any *already*
scanned txs in the wallet, then the wallet will also query the
daemon for all the *higher* txs as well. This is likely
unexpected behavior to a caller, and so to protect a caller from
revealing txid's to an untrusted daemon in an unexpected way,
require the daemon be trusted.

src/wallet/node_rpc_proxy.cpp

@@ -328,4 +328,23 @@ boost::optional<std::string> NodeRPCProxy::get_transactions(const std::vector<cr
   return boost::optional<std::string>();
 }
 
+boost::optional<std::string> NodeRPCProxy::get_block_header_by_height(uint64_t height, cryptonote::block_header_response &block_header)
+{
+  if (m_offline)
+    return boost::optional<std::string>("offline");
+
+  cryptonote::COMMAND_RPC_GET_BLOCK_HEADER_BY_HEIGHT::request req_t = AUTO_VAL_INIT(req_t);
+  cryptonote::COMMAND_RPC_GET_BLOCK_HEADER_BY_HEIGHT::response resp_t = AUTO_VAL_INIT(resp_t);
+  req_t.height = height;
+
+  {
+    const boost::lock_guard<boost::recursive_mutex> lock{m_daemon_rpc_mutex};
+    bool r = net_utils::invoke_http_json_rpc("/json_rpc", "getblockheaderbyheight", req_t, resp_t, m_http_client, rpc_timeout);
+    RETURN_ON_RPC_RESPONSE_ERROR(r, epee::json_rpc::error{}, resp_t, "getblockheaderbyheight");
+  }
+
+  block_header = std::move(resp_t.block_header);
+  return boost::optional<std::string>();
+}
+
 }