core: cache block template where possible

This avoids constant rechecking of the same things each time
a miner asks for the block template. The tx pool maintains
a cookie to allow users to detect when the pool state changed,
which means the block template needs rebuilding.

src/cryptonote_core/tx_pool.h

@@ -362,6 +362,13 @@ namespace cryptonote
      */
     size_t validate(uint8_t version);
 
+     /**
+      * @brief return the cookie
+      *
+      * @return the cookie
+      */
+    uint64_t cookie() const { return m_cookie; }
+
     /**
      * @brief get the cumulative txpool size in bytes
      *
@@ -549,6 +556,8 @@ private:
     //!< container for transactions organized by fee per size and receive time
     sorted_tx_container m_txs_by_fee_and_receive_time;
 
+    std::atomic<uint64_t> m_cookie; //!< incremented at each change
+
     /**
      * @brief get an iterator to a transaction in the sorted container
      *