blockchain: fix block template creation race

If two create_block_template are called at nearly the same time,
and a block is added at nearly the same time, this could happen:

- the blockchain top block is B0
- thread 1 enters create_block_template, takes blockchain lock
- thread 1 creates a fresh block referencing prev block B0
- thread 1 releases blockchain lock
- thread 0 adds a new block
- thread 0 enters create_block_template
- thread 0 updates block template
- thread 1 takes txpool lock and continues creating block template
- thread 1 overwrites block template with previous data

src/cryptonote_core/blockchain.cpp

@@ -1264,7 +1264,10 @@ bool Blockchain::create_block_template(block& b, const account_public_address& m
   uint64_t already_generated_coins;
   uint64_t pool_cookie;
 
-  CRITICAL_REGION_BEGIN(m_blockchain_lock);
+  m_tx_pool.lock();
+  const auto txpool_unlocker = epee::misc_utils::create_scope_leave_handler([&]() { m_tx_pool.unlock(); });
+  CRITICAL_REGION_LOCAL(m_blockchain_lock);
+
   height = m_db->height();
   if (m_btc_valid) {
     // The pool cookie is atomic. The lack of locking is OK, as if it changes
@@ -1300,8 +1303,6 @@ bool Blockchain::create_block_template(block& b, const account_public_address& m
   median_weight = m_current_block_cumul_weight_limit / 2;
   already_generated_coins = m_db->get_block_already_generated_coins(height - 1);
 
-  CRITICAL_REGION_END();
-
   size_t txs_weight;
   uint64_t fee;
   if (!m_tx_pool.fill_block_template(b, median_weight, already_generated_coins, txs_weight, fee, expected_reward, m_hardfork->get_current_version()))
@@ -1312,7 +1313,6 @@ bool Blockchain::create_block_template(block& b, const account_public_address& m
 #if defined(DEBUG_CREATE_BLOCK_TEMPLATE)
   size_t real_txs_weight = 0;
   uint64_t real_fee = 0;
-  CRITICAL_REGION_BEGIN(m_tx_pool.m_transactions_lock);
   for(crypto::hash &cur_hash: b.tx_hashes)
   {
     auto cur_res = m_tx_pool.m_transactions.find(cur_hash);
@@ -1356,7 +1356,7 @@ bool Blockchain::create_block_template(block& b, const account_public_address& m
   {
     LOG_ERROR("Creating block template: error: wrongly calculated fee");
   }
-  CRITICAL_REGION_END();
+
   MDEBUG("Creating block template: height " << height <<
       ", median weight " << median_weight <<
       ", already generated coins " << already_generated_coins <<