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

This will assist in a DB resize check.

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -1820,7 +1820,7 @@ bool BlockchainLMDB::has_key_image(const crypto::key_image& img) const
   return false;
 }
 
-void BlockchainLMDB::batch_start()
+void BlockchainLMDB::batch_start(uint64_t batch_num_blocks)
 {
   LOG_PRINT_L3("BlockchainLMDB::" << __func__);
   if (! m_batch_transactions)

src/blockchain_db/lmdb/db_lmdb.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();