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.

1 files changed, 8 insertions, 0 deletions

diff --git a/tokio/CHANGELOG.md b/tokio/CHANGELOG.md
index b748408e..68e2af65 100644
--- a/tokio/CHANGELOG.md
+++ b/tokio/CHANGELOG.md
@@ -1,3 +1,11 @@
+# 0.2.16 (April 3, 2020)
+
+### Fixes
+
+- sync: fix a regression where `Mutex`, `Semaphore`, and `RwLock` futures no
+  longer implement `Sync` (#2375)
+
+
 # 0.2.15 (April 2, 2020)
 
 ### Fixes