[leo_storage] When removing a large size object, a high-latency issue can be happened

[yosukehara]

I/we need to improve delete_objects_under_dir/1 function. After modifying this function, we'll be testing at our LeoFS environment.

[yosukehara]

I already fixed this issue two weeks ago, then we need to test this on our environment.

[mocchira]

Done tests.