core: lock incoming tx lock when checking the txpool and chain

This gets rid of an innocuous race trying to add the same tx
twice to the txpool

src/cryptonote_core/cryptonote_core.cpp

@@ -676,6 +676,7 @@ namespace cryptonote
   bool core::handle_incoming_txs(const std::list<blobdata>& tx_blobs, std::vector<tx_verification_context>& tvc, bool keeped_by_block, bool relayed, bool do_not_relay)
   {
     TRY_ENTRY();
+    CRITICAL_REGION_LOCAL(m_incoming_tx_lock);
 
     struct result { bool res; cryptonote::transaction tx; crypto::hash hash; crypto::hash prefix_hash; bool in_txpool; bool in_blockchain; };
     std::vector<result> results(tx_blobs.size());