Optionally allow DB to know expected number of blocks at batch transaction start

This will assist in a DB resize check.

src/blockchain_db/berkeleydb/db_bdb.h

@@ -191,7 +191,7 @@ public:
                             );
 
   virtual void set_batch_transactions(bool batch_transactions);
-  virtual void batch_start();
+  virtual void batch_start(uint64_t batch_num_blocks=0);
   virtual void batch_commit();
   virtual void batch_stop();
   virtual void batch_abort();