Faster shutdown of pgxpool.Pool background goroutines

When a pool is closed, some background goroutines may be left open,
particularly for health checks as detailed in #1641. Two specific
examples have been refactored here to avoid a blocking sleep and instead
also select on the pool being closed to potentially return/continue
sooner.

pgxpool/pool.go

@@ -384,10 +384,19 @@ func (p *Pool) triggerHealthCheck() {
 	go func() {
 		// Destroy is asynchronous so we give it time to actually remove itself from
 		// the pool otherwise we might try to check the pool size too soon
-		time.Sleep(500 * time.Millisecond)
+		t := time.NewTimer(500 * time.Millisecond)
+
 		select {
-		case p.healthCheckChan <- struct{}{}:
-		default:
+		case <-p.closeChan:
+			// If the pool is already closed, stop the timer and return:
+			if !t.Stop() {
+				<-t.C
+			}
+		case <-t.C:
+			select {
+			case p.healthCheckChan <- struct{}{}:
+			default:
+			}
 		}
 	}()
 }
@@ -408,6 +417,9 @@ func (p *Pool) backgroundHealthCheck() {
 }
 
 func (p *Pool) checkHealth() {
+	ticker := time.NewTicker(500 * time.Millisecond)
+	defer ticker.Stop()
+
 	for {
 		// If checkMinConns failed we don't destroy any connections since we couldn't
 		// even get to minConns
@@ -424,7 +436,7 @@ func (p *Pool) checkHealth() {
 		select {
 		case <-p.closeChan:
 			return
-		case <-time.After(500 * time.Millisecond):
+		case <-ticker.C:
 		}
 	}
 }