fix: free memory used by proposals in firewood (#1173)

Author: demosdemon

triedb/firewood/database.go

@@ -378,10 +378,15 @@ func (db *Database) Close() error {
 	db.proposalLock.Lock()
 	defer db.proposalLock.Unlock()
 
-	// We don't need to explicitly dereference the proposals, since they will be cleaned up
-	// within the firewood close method.
+	// before closing, we must deference any outstanding proposals to free the
+	// memory owned by firewood (outside of go's memory management)
+	for _, pCtx := range db.proposalTree.Children {
+		db.dereference(pCtx)
+	}
+
 	db.proposalMap = nil
 	db.proposalTree.Children = nil
+
 	// Close the database
 	return db.fwDisk.Close()
 }