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).

Author: sdaftuar

src/net_processing.cpp

@@ -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;