TSQL Formatter Service (#229)

- TSqlFormatterService with support for formatting document and text range inside document
- Settings support for all formatting options.
- Extensibility support so that the service can be initialized using MEF extensibility, and can find all necessary TSqlFormatters using the same process

Fix Initialize request error on startup
- Messages were being read from the input channel before all request handlers were registered
- In particular, the Initialize request which is key for any server to talk to the client was getting lost because the message reader thread begins consuming, and we take an extra few hundred milliseconds due to MEF startup before we register the handler
- The solution is to initialize the message handler so request handlers can register, but not actually start processing incoming messages until all handers are ready. This is a safer way to go and should improve reliability overall

Improvements from internal prototype:
- Normalizing baselines to handle the line ending differences on Mac & Linux vs. Windows
- Significantly shortened most lines by implementing base class methods to wrap common objects from Visitor.Context and removing unnecessary "this." syntax
- Refactored the SqlCommonTableExpressionFormatter and related classes to reduce code count significantly. This provides a pattern to follow when refactoring other classes for similar clarity. It's likely a lot of common logic could be found and reused across these.
- Reduced overall code size by adding utility methods

src/Microsoft.SqlTools.ServiceLayer/Extensibility/IMultiServiceProvider.cs

@@ -0,0 +1,104 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Microsoft.SqlTools.ServiceLayer.Utility;
+
+namespace Microsoft.SqlTools.ServiceLayer.Extensibility
+{
+    public interface IMultiServiceProvider
+    {
+        /// <summary>
+        /// Gets a service of a specific type. It is expected that only 1 instance of this type will be
+        /// available
+        /// </summary>
+        /// <typeparam name="T">Type of service to be found</typeparam>
+        /// <returns>Instance of T or null if not found</returns>
+        /// <exception cref="InvalidOperationException">The input sequence contains more than one element.-or-The input sequence is empty.</exception>
+        T GetService<T>();
+
+        /// <summary>
+        /// Gets a service of a specific type. The first service matching the specified filter will be returned
+        /// available
+        /// </summary>
+        /// <typeparam name="T">Type of service to be found</typeparam>
+        /// <param name="filter">Filter to use in </param>
+        /// <returns>Instance of T or null if not found</returns>
+        /// <exception cref="InvalidOperationException">The input sequence contains more than one element.-or-The input sequence is empty.</exception>
+        T GetService<T>(Predicate<T> filter);
+
+        /// <summary>
+        /// Gets multiple services of a given type
+        /// </summary>
+        /// <typeparam name="T"></typeparam>
+        /// <returns>An enumerable of matching services</returns>
+        IEnumerable<T> GetServices<T>();
+
+        /// <summary>
+        /// Gets multiple services of a given type, where they match a filter
+        /// </summary>
+        /// <typeparam name="T"></typeparam>
+        /// <param name="filter"></param>
+        /// <returns></returns>
+        IEnumerable<T> GetServices<T>(Predicate<T> filter);
+    }
+    
+    
+    public abstract class ServiceProviderBase : IMultiServiceProvider
+    {
+
+        public T GetService<T>()
+        {
+            return GetServices<T>().SingleOrDefault();
+        }
+
+        public T GetService<T>(Predicate<T> filter)
+        {
+            Validate.IsNotNull(nameof(filter), filter);
+            return GetServices<T>().Where(t => filter(t)).SingleOrDefault();
+        }
+
+        public IEnumerable<T> GetServices<T>(Predicate<T> filter)
+        {
+            Validate.IsNotNull(nameof(filter), filter);
+            return GetServices<T>().Where(t => filter(t));
+        }
+
+        public virtual IEnumerable<T> GetServices<T>()
+        {
+            var services = GetServicesImpl<T>();
+            if (services == null)
+            {
+                return Enumerable.Empty<T>();
+            }
+
+            return services.Select(t =>
+            {
+                InitComposableService(t);
+                return t;
+            });
+        }
+
+        private void InitComposableService<T>(T t)
+        {
+            IComposableService c = t as IComposableService;
+            if (c != null)
+            {
+                c.SetServiceProvider(this);
+            }
+        }
+        
+        /// <summary>
+        /// Gets all services using the build in implementation
+        /// </summary>
+        /// <typeparam name="T"></typeparam>
+        /// <returns></returns>
+        protected abstract IEnumerable<T> GetServicesImpl<T>();
+
+    }
+
+}