When `rb_concurrent_set_foreach_with_replace` deletes entries from a
concurrent set, it should increment the `deleted_entries` field, too.

When `rb_concurrent_set_find_or_insert` successfully deletes an entry
that points to a garbage object, we should increment `deleted_entries`,
too.  We must increment atomcially because another thread may be
deleting another entry concurrently.  For this, we change
`deleted_entries` to `rb_atomic_t`.