Use step_or_park from Timely (#70)

This is the first step to remediate #51 without running the chance of accidentally slowing down useful work.

* Add until_next() method to scheduler
* Use step_or_park in server loop

server/src/main.rs

@@ -700,8 +700,10 @@ fn main() {
             }
 
             // ensure work continues, even if no queries registered,
-            // s.t. the sequencer continues issuing commands
-            worker.step();
+            // s.t. the sequencer continues issuing commands.
+            // If there is nothing to do, park `until_next()` activation
+            // in scheduler is ready.
+            worker.step_or_park(Some(server.scheduler.borrow().until_next().unwrap_or(Duration::from_millis(100))));
 
             server.context.internal.advance().expect("failed to advance domain");
 

src/server/scheduler.rs

@@ -35,6 +35,21 @@ impl Scheduler {
             false
         }
     }
+    /// Returns the duration until the next activation in `activator_queue`
+    /// from `now()`. If no activations are present returns None.
+    pub fn until_next(&self) -> Option<Duration> {
+        if let Some(ref timed_activator) = self.activator_queue.peek() {
+            // timed_activator.at.check_duration_since(Instant::now()).unwrap_or(Duration::from_millies(0))
+            let now = Instant::now();
+            if timed_activator.at > now {
+                Some(timed_activator.at.duration_since(now))
+            } else {
+                Some(Duration::from_millis(0))
+            }
+        } else {
+            None
+        }
+    }
 
     /// Schedule activation at the specified instant. No hard
     /// guarantees on when the activator will actually be triggered.