go reactive: Cancel context after each reactive Run

Summary: Some pieces of code will hold onto the context waiting for it
to be done (usually in a goroutine).  This means that all the objects
that are stored on the context are held until the original context is
marked as "done".  This would not happen until the websocket/http
request finishes.  So for continous requests we have a leak where each
intermediary context is holding onto the entire cache stack (and
anything else on the context) until we reset the websocket connection.

This commit adds a cancel to context for each reactive.Run.  This should
allow all code that depends on the context.Done() to be able to clear
between runs.

reactive/rerunner.go

@@ -369,7 +369,13 @@ func (r *Rerunner) run() {
 		time.Sleep(WriteThenReadDelay)
 	}
 	r.cache.cleanInvalidated()
-	ctx := context.WithValue(r.ctx, cacheKey{}, r.cache)
+
+	// Cancel the context passed to "run". Canceling the context ensures that
+	// libraries let go of the context when they might otherwise hold onto it long
+	// past this run.
+	ctx, cancel := context.WithCancel(r.ctx)
+	defer cancel()
+	ctx = context.WithValue(ctx, cacheKey{}, r.cache)
 	ctx = context.WithValue(ctx, dependencySetKey{}, &dependencySet{})
 
 	currentComputation, err := run(ctx, r.f)