Removed unnecessary unsafe impl of Send and Sync

[peter-lyons-kehl]

Hi Kamil,

1. I didn't check your GIT commits history, but I believe that initially you may have had ShardedCache defined as pub struct ShardedCache<V> {..., or pub struct ShardedCache<V: Clone> {... as that is, without V being bound by Send+Sync; but later you changed it to include V to be bound by `Send+Sync?

Either way, good learning: While having a struct/enum/union defined with the least restricting bounds is a common and recommended practice, unfortunately, it doesn't auto infer Send and Sync, hence there would be a need for unsafe impl. But since the ShardedCache does have Send+Sync bound on V, Send and Sync are auto-inferred.

2. The second commit in this PR adds a "test" that ShardedCache is indeed Sync+Send. I do NOT think that such a test is necessary, but if you do want to include it, do so. Maybe you had unsafe impl so that it would be clear/"guaranteed" that Send and Sync were intentional. This test could indicate it instead.

But, if you don't add this test, then Send and Sync are required by other parts of the code. For example, when I changed ShardedCache NOT to make it Send+Sync by (shortened git diff here):

pub struct ShardedCache<V: Clone + Send + Sync> {
     shards: Vec<CacheShard<V>>,
     shard_mask: usize,
     stats: CacheStats,
+    rc: Option<std::rc::Rc<()>>
 }

(and I've modified the new constructor accordingly), then many other parts of the code failed to build, like:

error: future cannot be sent between threads safely
    --> src/runtime.rs:1156:10
     |
1156 |         .on_upgrade(move |socket| handle_live_socket(socket, mux))
     |          ^^^^^^^^^^ future returned by `handle_live_socket` is not `Send`

So, it should be clear to anyone that Send+Sync is intentional.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes

[Raezil]

Hi, thank you for your contribution, I will check it later.