Rewrite mpsc::shared

Previous version had too much state: `cnt`, `steals`, `port_dropped`
fields, and it's too hard to consistently update all of them during
upgrade.  I tried to fix issue rust-lang#39364, but there are too many corner
cases.  In this version all of these fields removed, and shared
state is basically managed by two fields: `queue` and `to_wait`.

`to_wake` field now stores both `SignalToken` and "disconnected"
flag.

All tests still pass, and bug rust-lang#39364 no longer reproduces.

Patch includes a couple of stress tests.

This version should have the same performance characteristics,
because roughly the same number of atomics and wait/notify operations
involved.

src/libstd/sync/mpsc/mpsc_queue.rs

@@ -114,6 +114,19 @@ impl<T> Queue<T> {
             if self.head.load(Ordering::Acquire) == tail {Empty} else {Inconsistent}
         }
     }
+
+    pub fn can_pop(&self) -> bool {
+        unsafe {
+            let tail = *self.tail.get();
+            let next = (*tail).next.load(Ordering::Acquire);
+
+            if !next.is_null() {
+                true
+            } else {
+                self.head.load(Ordering::Acquire) != tail
+            }
+        }
+    }
 }
 
 impl<T> Drop for Queue<T> {