task: fix panic when dropping LocalSet

it turns out that the `Scheduler::release` method on `LocalSet`'s
`Scheduler` *is* called, when the  `Scheduler` is dropped with tasks
still running. Currently, that method is `unreachable!`, which means
that dropping a `LocalSet` with tasks running will panic.

This commit fixes the panic, by pushing released tasks to
`pending_drop`. This is the same as `BasicScheduler`.

Fixes #1842

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

tokio/src/task/local.rs

@@ -345,8 +345,9 @@ impl Schedule for Scheduler {
         }
     }
 
-    fn release(&self, _: Task<Self>) {
-        unreachable!("tasks should only be completed locally")
+    fn release(&self, task: Task<Self>) {
+        // This will be called when dropping the local runtime.
+        self.pending_drop.push(task);
     }
 
     fn release_local(&self, task: &Task<Self>) {