Reroutes HttpZipkinSpanReporter to use zipkin.reporter.AsyncReporter (#409)

AsyncReporter is a more robust version of what we were doing before.
Notably, it can give a memory threshold instead of span count for the
backlog. This change ports to use AsyncReporter internally.

See https://github.com/openzipkin/zipkin-reporter-java#asyncreporter

benchmarks/pom.xml

@@ -163,6 +163,9 @@
 			<snapshots>
 				<enabled>true</enabled>
 			</snapshots>
+			<releases>
+				<enabled>false</enabled>
+			</releases>
 		</repository>
 		<repository>
 			<id>spring-milestones</id>
@@ -172,14 +175,6 @@
 				<enabled>false</enabled>
 			</snapshots>
 		</repository>
-		<repository>
-			<id>spring-staging</id>
-			<name>Spring Staging</name>
-			<url>https://repo.spring.io/libs-staging-local/</url>
-			<snapshots>
-				<enabled>false</enabled>
-			</snapshots>
-		</repository>
 		<repository>
 			<id>spring-releases</id>
 			<name>Spring Releases</name>