ts/executor: clear the reactor instead of closing it...

... so that it can be reused on current thread for subsequent
Scheduler instantiations (e.g. block_on) without the need to
reallocate internal data structures.

generic/threadshare/src/runtime/executor/reactor.rs

@@ -123,9 +123,22 @@ impl Reactor {
         })
     }
 
-    pub fn close() {
+    /// Clears the `Reactor`.
+    ///
+    /// It will be ready for reuse on current thread without reallocating.
+    pub fn clear() {
         let _ = CURRENT_REACTOR.try_with(|cur_reactor| {
-            *cur_reactor.borrow_mut() = None;
+            cur_reactor.borrow_mut().as_mut().map(|reactor| {
+                reactor.ticker = AtomicUsize::new(0);
+                reactor.wakers.clear();
+                reactor.sources.clear();
+                reactor.events.clear();
+                reactor.timers.clear();
+                reactor.after_timers.clear();
+                while !reactor.timer_ops.is_empty() {
+                    let _ = reactor.timer_ops.pop();
+                }
+            })
         });
     }
 

generic/threadshare/src/runtime/executor/scheduler.rs

@@ -242,7 +242,7 @@ impl Scheduler {
             context_name,
         );
 
-        Reactor::close();
+        Reactor::clear();
 
         let _ = CURRENT_SCHEDULER.try_with(|cur_scheduler| {
             *cur_scheduler.borrow_mut() = None;