sync: reduce memory size of watch::Receiver (#2191)
sync: reduce memory size of watch::Receiver (#2191)

This reduces the `mem::size_of::<watch::Receiver>()` from 4 words to 2.

- The `id` is now the pointer of the `Arc<WatchInner>`.
- The `ver` is moved into the `WatchInner`.

tokio-rs/tokio,tokio-rs/tokio