Use WeakHashMap in IdleConenctionReaper (#4087)

* Use WeakHashMap in IdleConenctionReaper to not prevent connection manager from getting GC'd

* Checkstyle fix

Author: zoewangg

.changes/next-release/feature-AWSSDKforJavav2-15df2d6.json

@@ -0,0 +1,6 @@
+{
+    "type": "feature",
+    "category": "AWS SDK for Java v2",
+    "contributor": "",
+    "description": "`IdleConnectionReaper` now does not prevent `HttpClientConnectionManager` from getting GC'd in the case where an SDK client is created per request and not closed."
+}

http-clients/apache-client/src/main/java/software/amazon/awssdk/http/apache/internal/conn/IdleConnectionReaper.java

@@ -16,8 +16,9 @@
 package software.amazon.awssdk.http.apache.internal.conn;
 
 import java.time.Duration;
+import java.util.Collections;
 import java.util.Map;
-import java.util.concurrent.ConcurrentHashMap;
+import java.util.WeakHashMap;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 import java.util.concurrent.TimeUnit;
@@ -48,7 +49,7 @@ public final class IdleConnectionReaper {
     private volatile ReaperTask reaperTask;
 
     private IdleConnectionReaper() {
-        this.connectionManagers =  new ConcurrentHashMap<>();
+        this.connectionManagers = Collections.synchronizedMap(new WeakHashMap<>());
 
         this.executorServiceSupplier = () -> {
             ExecutorService e = Executors.newSingleThreadExecutor(r -> {