Retrieve ID B+tree in seq iterator, if custom cmp function is given

- If custom comparison function for variable-length key is given,
ID B+tree is used instead of HB+trie. both _fdb_iterator_seq_next()
and _fdb_iterator_seq_prev() functions should retrieve ID B+tree for
duplication checks, if custom cmp function is assigned from users.

Change-Id: Ie12620cd9c598c72af2e735b0a0950188ed11e55