core: faster find_blockchain_supplement

Since this queries block heights for blocks that may or may not
exist, queries for non existing blocks would throw an exception,
and that would slow down the loop a lot. 7 seconds to go through
a 30 hash list.

Fix this by adding an optional return block height to block_exists
and using this instead. Actual errors will still throw an
exception.

This also cuts down on log exception spam.

src/blockchain_db/blockchain_db.h

@@ -736,10 +736,11 @@ public:
    * @brief checks if a block exists
    *
    * @param h the hash of the requested block
+   * @param height if non NULL, returns the block's height if found
    *
    * @return true of the block exists, otherwise false
    */
-  virtual bool block_exists(const crypto::hash& h) const = 0;
+  virtual bool block_exists(const crypto::hash& h, uint64_t *height = NULL) const = 0;
 
   /**
    * @brief fetches the block with the given hash