Partial fix #1876, check thread in block_txn_start()

Don't allow use of existing batch txn if it's from the wrong thread

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -2503,7 +2503,8 @@ void BlockchainLMDB::block_txn_start(bool readonly)
       throw0(DB_ERROR_TXN_START(lmdb_error("Failed to create a transaction for the db: ", mdb_res).c_str()));
     }
     memset(&m_wcursors, 0, sizeof(m_wcursors));
-  }
+  } else if (m_writer != boost::this_thread::get_id())
+    throw0(DB_ERROR_TXN_START((std::string("Attempted to start new write txn when batch txn already exists in ")+__FUNCTION__).c_str()));
 }
 
 void BlockchainLMDB::block_txn_stop()