trie: add finalizer to free memory from cleans fastcache

Author: tsahee

trie/database.go

@@ -301,9 +301,17 @@ func NewDatabaseWithConfig(diskdb ethdb.KeyValueStore, config *Config) *Database
 		}},
 		preimages: preimage,
 	}
+	runtime.SetFinalizer(db, (*Database).finalizer)
 	return db
 }
 
+// must call Reset() to reclaim memory used by fastcache
+func (db *Database) finalizer() {
+	if db.cleans != nil {
+		db.cleans.Reset()
+	}
+}
+
 // insert inserts a simplified trie node into the memory database.
 // All nodes inserted by this function will be reference tracked
 // and in theory should only used for **trie nodes** insertion.