Introduce the `drop_unused_requests` option (False by default). If True, it will force the `Cassette` saving operation with only played old interactions and new ones if they exist. As a result, unused old requests are dropped.

Add `_old_interactions`, `_played_interactions` and `_new_interactions()`.  The `_old_interactions` are previously recorded interactions loaded from Cassette files. The `_played_interactions` is a set of old interactions that were marked as played.  A new interaction is a tuple (request, response) in `self.data` that is not in `_old_interactions` list.