db_lmdb: enable batch transactions by default

src/blockchain_db/lmdb/db_lmdb.h

@@ -157,7 +157,7 @@ struct mdb_txn_safe
 class BlockchainLMDB : public BlockchainDB
 {
 public:
-  BlockchainLMDB(bool batch_transactions=false);
+  BlockchainLMDB(bool batch_transactions=true);
   ~BlockchainLMDB();
 
   virtual void open(const std::string& filename, const int mdb_flags=0);