Fixes issues in HealthCheckServiceInstanceListSupplier (#685)

- WebClient response leaks
- potential Scheduled task leak
- rework polling to plain reactor operators
- remove non atomic operations on ServiceInstance lists

related to gh-629
fixes issues from gh-683

spring-cloud-loadbalancer/pom.xml

@@ -88,5 +88,10 @@
 			<artifactId>reactor-test</artifactId>
 			<scope>test</scope>
 		</dependency>
+		<dependency>
+			<groupId>org.awaitility</groupId>
+			<artifactId>awaitility</artifactId>
+			<scope>test</scope>
+		</dependency>
 	</dependencies>
 </project>