feature/edit/subset (#283)

* Changing query/subset API to only use Result on success, Error on error

* Creating an interservice API for getting query result subsets

* Updates to subset API

* RowStartIndex is now long
* Output of query/subset is a 2D array of DbCellValue
* Adding LongSkip method to LongList to allow skipping ahead by longs
* Moving LongList back to ServiceLayer utilities. Move refactoring

* Stubbing out request for edit/subset

* Initial implementation of getting edit rows

* Unit tests for RowEdit and RowDelete .GetEditRow

* Fixing major bugs in LongList implementation, adding much more thorough tests

* Adding some more unit tests and fixes to make unit tests pass

* Fixing comment

src/Microsoft.SqlTools.ServiceLayer/EditData/Contracts/EditSubsetRequest.cs

@@ -0,0 +1,52 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using Microsoft.SqlTools.Hosting.Protocol.Contracts;
+
+namespace Microsoft.SqlTools.ServiceLayer.EditData.Contracts
+{
+    /// <summary>
+    /// Parameters for a subset retrieval request
+    /// </summary>
+    public class EditSubsetParams : SessionOperationParams
+    {
+        /// <summary>
+        /// Beginning index of the rows to return from the selected resultset. This index will be
+        /// included in the results.
+        /// </summary>
+        public long RowStartIndex { get; set; }
+
+        /// <summary>
+        /// Number of rows to include in the result of this request. If the number of the rows
+        /// exceeds the number of rows available after the start index, all available rows after
+        /// the start index will be returned.
+        /// </summary>
+        public int RowCount { get; set; }
+    }
+
+    /// <summary>
+    /// Parameters for the result of a subset retrieval request
+    /// </summary>
+    public class EditSubsetResult
+    {
+        /// <summary>
+        /// The number of rows returned from result set, useful for determining if less rows were
+        /// returned than requested.
+        /// </summary>
+        public int RowCount { get; set; }
+
+        /// <summary>
+        /// The requested subset of rows, with information about whether or not the rows are dirty
+        /// </summary>
+        public EditRow[] Subset { get; set; }
+    }
+
+    public class EditSubsetRequest
+    {
+        public static readonly
+            RequestType<EditSubsetParams, EditSubsetResult> Type =
+            RequestType<EditSubsetParams, EditSubsetResult>.Create("edit/subset");
+    }
+}