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/lmdb/db_lmdb.h

@@ -168,7 +168,7 @@ public:
 
   virtual void unlock();
 
-  virtual bool block_exists(const crypto::hash& h) const;
+  virtual bool block_exists(const crypto::hash& h, uint64_t *height = NULL) const;
 
   virtual block get_block(const crypto::hash& h) const;