Blockchain: Omit verbose time stats messages by default

This is already the default for the daemon, but by checking a command
line argument and calling a Blockchain member function setter.

Initialize the variable to false so it's not dependent on an external
command-line argument check. This allows utilities like
blockchain_import to have a reasonable default without code changes.

src/cryptonote_core/blockchain.cpp

@@ -101,7 +101,7 @@ static const uint64_t testnet_hard_fork_version_1_till = 624633;
 //------------------------------------------------------------------
 Blockchain::Blockchain(tx_memory_pool& tx_pool) :
   m_db(), m_tx_pool(tx_pool), m_hardfork(NULL), m_timestamps_and_difficulties_height(0), m_current_block_cumul_sz_limit(0), m_is_in_checkpoint_zone(false),
-  m_is_blockchain_storing(false), m_enforce_dns_checkpoints(false), m_max_prepare_blocks_threads(4), m_db_blocks_per_sync(1), m_db_sync_mode(db_async), m_fast_sync(true), m_sync_counter(0)
+  m_is_blockchain_storing(false), m_enforce_dns_checkpoints(false), m_max_prepare_blocks_threads(4), m_db_blocks_per_sync(1), m_db_sync_mode(db_async), m_fast_sync(true), m_show_time_stats(false), m_sync_counter(0)
 {
   LOG_PRINT_L3("Blockchain::" << __func__);
 }