rt: use release ordering in `drop_join_handle_fast` (#2044)
rt: use release ordering in `drop_join_handle_fast` (#2044)

Previously acquire operations reading a value written by a successful
CAS in `drop_join_handle_fast` did not synchronize with it. The CAS
wasn't guaranteed to happen before the task deallocation, and so
created a data race between the two.

Use release success ordering to ensure synchronization.
tokio-rs/tokio,tokio-rs/tokio