RGW/multisite: bucket_list_result object provides a method to reset its entries

RGW/multisite: bucket_list_result object provides a method to reset its entries

Add a new method `reset_entries()` to the `bucket_list_result` struct
that clears the list of entries and resets the truncated flag.

This would be used to enhance the re-use cases to avoid accessing stale
entries or truncated flag.

Signed-off-by: Oguzhan Ozmen <oozmen@bloomberg.net>
(cherry picked from commit 911d0cb2bce2a613657a6aebcb9724cabc23d6eb)
resolves rhbz#2412220
(cherry picked from commit ac270428b90cc411074467db4c5f0dfd2e9e1e59)