Stopping sessions when handling stop requests (#627)

* Dropping profiler session on stop request

* Changes to IXEventSession to simplify dropping sessions

* Stop sessions instead of dropping, disable flaky tests

src/Microsoft.SqlTools.ServiceLayer/Profiler/IXEventSession.cs

@@ -14,5 +14,10 @@ namespace Microsoft.SqlTools.ServiceLayer.Profiler
         /// Reads XEvent XML from the default session target
         /// </summary>
         string GetTargetXml();
+
+        /// <summary>
+        /// Stops XEvent session
+        /// </summary>
+        void Stop();
     }
 }