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/berkeleydb/db_bdb.cpp

@@ -1235,7 +1235,7 @@ void BlockchainBDB::unlock()
     check_open();
 }
 
-bool BlockchainBDB::block_exists(const crypto::hash& h) const
+bool BlockchainBDB::block_exists(const crypto::hash& h, uint64_t *height) const
 {
     LOG_PRINT_L3("BlockchainBDB::" << __func__);
     check_open();
@@ -1251,6 +1251,9 @@ bool BlockchainBDB::block_exists(const crypto::hash& h) const
     else if (get_result)
         throw0(DB_ERROR("DB error attempting to fetch block index from hash"));
 
+    if (height)
+      *height = get_result - 1;
+
     return true;
 }