Provide safe method to delete a nested bucket during iteration

[davecgh]

Currently, there is no safe way to delete buckets during iteration. This is because it's not safe to delete/add new keys or buckets during ForEach iteration, the Delete method on a Cursor can't be used for buckets, and there is no DeleteBucket method on a Cursor. As a result, code to remove all nested buckets and keys in a given bucket ends up looking like the following:

        // Remove all the keys using a cursor while also generating a
        // list of buckets.  It's not safe to remove keys during ForEach
        // iteration nor is it safe to remove buckets during cursor
        // iteration, so this dual approach is needed.
        var bucketNames [][]byte
        cursor := bucket.Cursor()
        for k, v := cursor.First(); k != nil; k, v = cursor.Next() {
            if v != nil {
                if err := cursor.Delete(); err != nil {
                    return err
                }
            } else {
                bucketNames = append(bucketNames, k)
            }
        }

        // Remove the buckets.
        for _, k := range bucketNames {
            if err := bucket.DeleteBucket(k); err != nil {
                return err
            }
        }

I would suggest either adding a DeleteBucket method to the Cursor type which provides the same safety guarantee Delete provides for keys during cursor iteration, or modifying Delete to work with both keys and buckets.