ARROW-6795: [C#] Fix for reading large (2GB+) files

It seems that trying to read larger than 2GB+ files will blow up.

As long as the record batches are less than 2GB (the max size of span) there should be no problem reading a large file

Closes #5412 from abbotware/Fix-For-Large-Files and squashes the following commits:

898d556 <Eric Erhardt> Respond to PR feedback.
a556ac2 <Anthony Abate> fix for reading large (2GB+) files

Lead-authored-by: Anthony Abate <anthony.abate@gmail.com>
Co-authored-by: Eric Erhardt <eric.erhardt@microsoft.com>
Signed-off-by: Micah Kornfield <emkornfield@gmail.com>

csharp/src/Apache.Arrow/Ipc/ArrowFileReaderImplementation.cs

@@ -33,11 +33,6 @@ internal sealed class ArrowFileReaderImplementation : ArrowStreamReaderImplement
         /// </summary>
         private int _recordBatchIndex;
 
-        /// <summary>
-        /// Notes what byte position where the footer data is in the stream
-        /// </summary>
-        private int _footerStartPostion;
-
         private ArrowFooter _footer;
 
         public ArrowFileReaderImplementation(Stream stream, MemoryAllocator allocator, bool leaveOpen)
@@ -77,9 +72,9 @@ protected override async ValueTask ReadSchemaAsync()
 
             await ArrayPool<byte>.Shared.RentReturnAsync(footerLength, async (buffer) =>
             {
-                _footerStartPostion = (int)GetFooterLengthPosition() - footerLength;
+                long footerStartPostion = GetFooterLengthPosition() - footerLength;
 
-                BaseStream.Position = _footerStartPostion;
+                BaseStream.Position = footerStartPostion;
 
                 int bytesRead = await BaseStream.ReadFullBufferAsync(buffer).ConfigureAwait(false);
                 EnsureFullRead(buffer, bytesRead);
@@ -110,9 +105,9 @@ protected override void ReadSchema()
 
             ArrayPool<byte>.Shared.RentReturn(footerLength, (buffer) =>
             {
-                _footerStartPostion = (int)GetFooterLengthPosition() - footerLength;
+                long footerStartPostion = GetFooterLengthPosition() - footerLength;
 
-                BaseStream.Position = _footerStartPostion;
+                BaseStream.Position = footerStartPostion;
 
                 int bytesRead = BaseStream.ReadFullBuffer(buffer);
                 EnsureFullRead(buffer, bytesRead);