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.cpp

@@ -1595,7 +1595,7 @@ bool BlockchainBDB::has_key_image(const crypto::key_image& img) const
 // Ostensibly BerkeleyDB has batch transaction support built-in,
 // so the following few functions will be NOP.
 
-void BlockchainBDB::batch_start()
+void BlockchainBDB::batch_start(uint64_t batch_num_blocks)
 {
   LOG_PRINT_L3("BlockchainBDB::" << __func__);
 }