Use batch transactions when syncing

Faster throughput while avoiding corruption. I.e., makes
running with --db-sync-mode safe more tolerable.

src/blockchain_db/berkeleydb/db_bdb.cpp

@@ -1813,9 +1813,10 @@ 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(uint64_t batch_num_blocks)
+bool BlockchainBDB::batch_start(uint64_t batch_num_blocks)
 {
     LOG_PRINT_L3("BlockchainBDB::" << __func__);
+    return false;
 }
 
 void BlockchainBDB::batch_commit()

src/blockchain_db/berkeleydb/db_bdb.h

@@ -324,7 +324,7 @@ public:
                             );
 
   virtual void set_batch_transactions(bool batch_transactions);
-  virtual void batch_start(uint64_t batch_num_blocks=0);
+  virtual bool batch_start(uint64_t batch_num_blocks=0);
   virtual void batch_commit();
   virtual void batch_stop();
   virtual void batch_abort();

src/blockchain_db/blockchain_db.h

@@ -655,16 +655,17 @@ public:
    * been called.  In either case, it should end the batch and write to its
    * backing store.
    *
-   * If a batch is already in-progress, this function should throw a DB_ERROR.
-   * This exception may change in the future if it is deemed necessary to
-   * have a more granular exception type for this scenario.
+   * If a batch is already in-progress, this function must return false.
+   * If a batch was started by this call, it must return true.
    *
    * If any of this cannot be done, the subclass should throw the corresponding
    * subclass of DB_EXCEPTION
    *
    * @param batch_num_blocks number of blocks to batch together
+   *
+   * @return true if we started the batch, false if already started
    */
-  virtual void batch_start(uint64_t batch_num_blocks=0) = 0;
+  virtual bool batch_start(uint64_t batch_num_blocks=0) = 0;
 
   /**
    * @brief ends a batch transaction

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -2234,15 +2234,15 @@ bool BlockchainLMDB::for_all_outputs(std::function<bool(uint64_t amount, const c
 }
 
 // batch_num_blocks: (optional) Used to check if resize needed before batch transaction starts.
-void BlockchainLMDB::batch_start(uint64_t batch_num_blocks)
+bool BlockchainLMDB::batch_start(uint64_t batch_num_blocks)
 {
   LOG_PRINT_L3("BlockchainLMDB::" << __func__);
   if (! m_batch_transactions)
     throw0(DB_ERROR("batch transactions not enabled"));
   if (m_batch_active)
-    throw0(DB_ERROR("batch transaction already in progress"));
+    return false;
   if (m_write_batch_txn != nullptr)
-    throw0(DB_ERROR("batch transaction already in progress"));
+    return false;
   if (m_write_txn)
     throw0(DB_ERROR("batch transaction attempted, but m_write_txn already in use"));
   check_open();
@@ -2268,6 +2268,7 @@ void BlockchainLMDB::batch_start(uint64_t batch_num_blocks)
   memset(&m_wcursors, 0, sizeof(m_wcursors));
 
   LOG_PRINT_L3("batch transaction: begin");
+  return true;
 }
 
 void BlockchainLMDB::batch_commit()

src/blockchain_db/lmdb/db_lmdb.h

@@ -245,7 +245,7 @@ public:
                             );
 
   virtual void set_batch_transactions(bool batch_transactions);
-  virtual void batch_start(uint64_t batch_num_blocks=0);
+  virtual bool batch_start(uint64_t batch_num_blocks=0);
   virtual void batch_commit();
   virtual void batch_stop();
   virtual void batch_abort();