Adding Milliseconds to DateTime fields (#173)

This is a slightly larger change than anticipated due the difference between `DATETIME`, `DATETIME2`, and `DateTime`. The `DATETIME` type always uses 3 decimal points of a second, while the `DATETIME2` type has 7 (although since `DATETIME2(7)` is default in SSMS suggesting that it is a variable precision type). Regardless of the db type, the engine returns `DateTime` C# type. The db types are only made visible via the column info, via the numeric precision and numeric scale. My findings were as such:
`DATETIME `: Precision = 23, Scale = 3
`DATETIME2`: Precision = 255, Scale = 7

The scale corresponds neatly with the number of second decimal points to show. The buffer file writer was modified to store both the scale and the number of ticks. Then the buffer file reader was modified to read in the precision and the number of ticks and generate the ToString version of the DateTime to add "f" as many times as there is scale, which corresponds to milliseconds.

* Code for writing milliseconds of datetime/datetime2 columns

* Adding unit tests

* Fixing potential bug with datetime2(0)

test/Microsoft.SqlTools.ServiceLayer.Test/QueryExecution/DataStorage/ServiceBufferFileStreamReaderWriterTests.cs

@@ -8,7 +8,9 @@ using System.Collections.Generic;
 using System.Data.SqlTypes;
 using System.IO;
 using System.Text;
+using Microsoft.SqlTools.ServiceLayer.QueryExecution.Contracts;
 using Microsoft.SqlTools.ServiceLayer.QueryExecution.DataStorage;
+using Microsoft.SqlTools.ServiceLayer.Test.Utility;
 using Moq;
 using Xunit;
 
@@ -229,18 +231,21 @@ namespace Microsoft.SqlTools.ServiceLayer.Test.QueryExecution.DataStorage
             }
         }
 
-        [Fact]
-        public void DateTimeTest()
+        [Theory]
+        [InlineData(3)] // Scale 3 = DATETIME
+        [InlineData(7)] // Scale 7 = DATETIME2
+        public void DateTimeTest(int scale)
         {
-            // Setup: Create some test values
+            // Setup: Create some test values and a column with scale set
             // NOTE: We are doing these here instead of InlineData because DateTime values can't be written as constant expressions
+            DbColumnWrapper col = new DbColumnWrapper(new TestDbColumn("dbcol", scale));
             DateTime[] testValues = 
             {
                 DateTime.Now, DateTime.UtcNow, DateTime.MinValue, DateTime.MaxValue
             };
             foreach (DateTime value in testValues)
             {
-                VerifyReadWrite(sizeof(long) + 1, value, (writer, val) => writer.WriteDateTime(val), reader => reader.ReadDateTime(0));
+                VerifyReadWrite(sizeof(long) + sizeof(int) + 1, value, (writer, val) => writer.WriteDateTime(col, val), reader => reader.ReadDateTime(0));
             }
         }