message: Add an explicit memory bound to m_tx_process_time
  Previously there was an implicit bound based on the handling of m_tx_announced,
  but that approach is error-prone (particularly if we start automatically
  removing things from that set).
diff: @@ -699,7 +699,9 @@ void UpdateTxRequestTime(const uint256& txid, int64_t request_time) EXCLUSIVE_LO
  void RequestTx(CNodeState* state, const uint256& txid, int64_t nNow) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
  {
  CNodeState::TxDownloadState& peer_download_state = state->m_tx_download;
  - if (peer_download_state.m_tx_announced.size() >= MAX_PEER_TX_ANNOUNCEMENTS || peer_download_state.m_tx_announced.count(txid)) {
  + if (peer_download_state.m_tx_announced.size() >= MAX_PEER_TX_ANNOUNCEMENTS ||
  + peer_download_state.m_tx_process_time.size() >= MAX_PEER_TX_ANNOUNCEMENTS ||
  + peer_download_state.m_tx_announced.count(txid)) {
  // Too many queued announcements from this peer, or we already have
  // this announcement
  return;
  