BlockchainDB/LMDB: Refactor block-scope DB txn handling for add block

Move block-scope txn start and stop from BlockchainLMDB to BlockchainDB.

src/blockchain_db/blockchain_db.cpp

@@ -99,6 +99,8 @@ uint64_t BlockchainDB::add_block( const block& blk
                                 , const std::vector<transaction>& txs
                                 )
 {
+  block_txn_start();
+
   TIME_MEASURE_START(time1);
   crypto::hash blk_hash = get_block_hash(blk);
   TIME_MEASURE_FINISH(time1);
@@ -125,9 +127,15 @@ uint64_t BlockchainDB::add_block( const block& blk
   TIME_MEASURE_FINISH(time1);
   time_add_transaction += time1;
 
+  // DB's new height based on this added block is only incremented after this
+  // function returns, so height() here returns the new previous height.
+  uint64_t prev_height = height();
+
+  block_txn_stop();
+
   ++num_calls;
 
-  return height();
+  return prev_height;
 }
 
 void BlockchainDB::pop_block(block& blk, std::vector<transaction>& txs)

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -2244,33 +2244,15 @@ uint64_t BlockchainLMDB::add_block(const block& blk, const size_t& block_size, c
     }
   }
 
-  mdb_txn_safe txn;
-  if (! m_batch_active)
-  {
-    if (auto mdb_res = mdb_txn_begin(m_env, NULL, 0, txn))
-      throw0(DB_ERROR(lmdb_error("Failed to create a transaction for the db: ", mdb_res).c_str()));
-    m_write_txn = &txn;
-  }
-
   uint64_t num_outputs = m_num_outputs;
   try
   {
     BlockchainDB::add_block(blk, block_size, cumulative_difficulty, coins_generated, txs);
-    if (! m_batch_active)
-    {
-      m_write_txn = NULL;
-
-      TIME_MEASURE_START(time1);
-      txn.commit();
-      TIME_MEASURE_FINISH(time1);
-      time_commit1 += time1;
-    }
   }
   catch (...)
   {
     m_num_outputs = num_outputs;
-    if (! m_batch_active)
-      m_write_txn = NULL;
+    block_txn_abort();
     throw;
   }