Create MS.SqlTools.Credentials project (#249)

* Initial credential service files

* Clean-up hostloader

* Update build scripts to build credentials archive

* Move hosting files to new assembly

* Add credentials files to MS.SqlTools.Credentials

* Remove duplicate files

* Update namespace in program.cs

* Fix test build breaks

* Update extensions visibility.

* Remove unused resource strings

* Add xproj files to SLN for appveyor builds

* Fix appveyor build break in test project

* Fix extensibility tests

* Fix various typos in latest iteration

* Add settings for Integration build

* Fix codecoverage.bat to use full pdb for new projects

* Fix bug when packing in folder with native images

* Fix typos in xproj

* Reset XLF to fix build.cmd

src/Microsoft.SqlTools.Hosting/Hosting/ServiceHostBase.cs

@@ -0,0 +1,47 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using System.Threading.Tasks;
+using Microsoft.SqlTools.ServiceLayer.Hosting.Contracts;
+using Microsoft.SqlTools.ServiceLayer.Hosting.Protocol;
+using Microsoft.SqlTools.ServiceLayer.Hosting.Protocol.Channel;
+
+namespace Microsoft.SqlTools.ServiceLayer.Hosting
+{
+    public abstract class ServiceHostBase : ProtocolEndpoint
+    {
+        private bool isStarted;
+        private TaskCompletionSource<bool> serverExitedTask;
+
+        protected ServiceHostBase(ChannelBase serverChannel) : 
+            base(serverChannel, MessageProtocolType.LanguageServer)
+        {
+        }
+
+        protected override Task OnStart()
+        {
+            // Register handlers for server lifetime messages
+            
+            this.SetEventHandler(ExitNotification.Type, this.HandleExitNotification);
+
+            return Task.FromResult(true);
+        }
+
+        private async Task HandleExitNotification(
+            object exitParams,
+            EventContext eventContext)
+        {
+            // Stop the server channel
+            await this.Stop();
+
+            // Notify any waiter that the server has exited
+            if (this.serverExitedTask != null)
+            {
+                this.serverExitedTask.SetResult(true);
+            }
+        }
+    }
+}
+