sync: ensure Mutex, RwLock, and Semaphore futures are Send + Sync (#2375)

Previously, the `Mutex::lock`, `RwLock::{read, write}`, and
`Semaphore::acquire` futures in `tokio::sync` implemented `Send + Sync`
automatically. This was by virtue of being implemented using a `poll_fn`
that only closed over `Send + Sync` types. However, this broke in
PR #2325, which rewrote those types using the new `batch_semaphore`.
Now, they await an `Acquire` future, which contains a `Waiter`, which
internally contains an `UnsafeCell`, and thus does not implement `Sync`.

Since removing previously implemented traits breaks existing code, this
inadvertantly caused a breaking change. There were tests ensuring that
the `Mutex`, `RwLock`, and `Semaphore` types themselves were `Send +
Sync`, but no tests that the _futures they return_ implemented those
traits.

I've fixed this by adding an explicit impl of `Sync` for the
`batch_semaphore::Acquire` future. Since the `Waiter` type held by this
struct is only accessed when borrowed mutably, it is safe for it to
implement `Sync`.

Additionally, I've added to the bounds checks for the effected
`tokio::sync` types to ensure that returned futures continue to
implement `Send + Sync` in the future.

tokio/Cargo.toml

@@ -8,12 +8,12 @@ name = "tokio"
 #   - README.md
 # - Update CHANGELOG.md.
 # - Create "v0.2.x" git tag.
-version = "0.2.15"
+version = "0.2.16"
 edition = "2018"
 authors = ["Tokio Contributors <[email protected]>"]
 license = "MIT"
 readme = "README.md"
-documentation = "https://docs.rs/tokio/0.2.15/tokio/"
+documentation = "https://docs.rs/tokio/0.2.16/tokio/"
 repository = "https://github.com/tokio-rs/tokio"
 homepage = "https://tokio.rs"
 description = """