Fix a memory leak on each HTTP request made to ROS

By default NSURLSession will keep itself alive indefinitely, and has to be
explicitly told to delete itself after it completes all enqueued tasks.

CHANGELOG.md

@@ -1,3 +1,17 @@
+x.y.z Release notes (yyyy-MM-dd)
+=============================================================
+### Enhancements
+* None.
+
+### Fixed
+* Fix a memory leak whenever Realm makes a HTTP(s) request to the Realm Object
+  Server (since 3.8.0, Issue [#6058](https://github.com/realm/realm-cocoa/issues/6058)).
+
+### Compatibility
+* File format: Generates Realms with format v9 (Reads and upgrades all previous formats)
+* Realm Object Server: 3.11.0 or later.
+* APIs are backwards compatible with all previous releases in the 3.x.y series.
+
 3.13.1 Release notes (2019-01-03)
 =============================================================
 

Realm/RLMNetworkClient.mm

@@ -425,6 +425,8 @@ + (void)sendRequestToEndpoint:(RLMSyncServerEndpoint *)endpoint
 
     // Add the request to a task and start it
     [[session dataTaskWithRequest:request] resume];
+    // Tell the session to destroy itself once it's done with the request
+    [session finishTasksAndInvalidate];
 }
 @end