ARROW-4543: [C#] Update Flat Buffers code to latest version

- Update FlatBuffers code to latest version.
- Mark all FlatBuffer types as internal.

Note: I didn't use the latest `.fbs` file version because it included the SparseTensor support. Using the latest `.fbs` change caused problems with the `Tensor` generation. I have a [question if the latest `.fbs` file is correct](apache/arrow#2546 (comment)).

/cc @chutchinson @stephentoub - FYI

Author: Eric Erhardt <eric.erhardt@microsoft.com>

Closes #3637 from eerhardt/UpdateFlatBufferCode and squashes the following commits:

15e0831c <Eric Erhardt> Adding how to update FlatBuffers code to the README
043ce7e2 <Eric Erhardt> ARROW-4543:  Update Flat Buffers code to latest version

Author: eerhardt

README.md

@@ -148,3 +148,11 @@ Build from the Apache Arrow project root.
 	dotnet test test/Apache.Arrow.Tests
 
 All build artifacts are placed in the **artifacts** folder in the project root.
+
+# Updating FlatBuffers code
+
+See https://google.github.io/flatbuffers/flatbuffers_guide_use_java_c-sharp.html for how to get the `flatc` executable.
+
+Run `flatc --csharp` on each `.fbs` file in the [format](../format) folder. And replace the checked in `.cs` files under [FlatBuf](src/Apache.Arrow/Flatbuf) with the generated files.
+
+Update the non-generated [FlatBuffers](src/Apache.Arrow/Flatbuf/FlatBuffers) `.cs` files with the files from the [google/flatbuffers repo](https://github.com/google/flatbuffers/tree/master/net/FlatBuffers).

src/Apache.Arrow/Flatbuf/Block.cs

@@ -8,7 +8,7 @@ namespace Apache.Arrow.Flatbuf
 using global::System;
 using global::FlatBuffers;
 
-public struct Block : IFlatbufferObject
+internal struct Block : IFlatbufferObject
 {
   private Struct __p;
   public ByteBuffer ByteBuffer { get { return __p.bb; } }

src/Apache.Arrow/Flatbuf/Buffer.cs

@@ -10,7 +10,7 @@ namespace Apache.Arrow.Flatbuf
 
 /// ----------------------------------------------------------------------
 /// A Buffer represents a single contiguous memory segment
-public struct Buffer : IFlatbufferObject
+internal struct Buffer : IFlatbufferObject
 {
   private Struct __p;
   public ByteBuffer ByteBuffer { get { return __p.bb; } }

src/Apache.Arrow/Flatbuf/DictionaryBatch.cs

@@ -14,7 +14,7 @@ namespace Apache.Arrow.Flatbuf
 /// There is one vector / column per dictionary, but that vector / column
 /// may be spread across multiple dictionary batches by using the isDelta
 /// flag
-public struct DictionaryBatch : IFlatbufferObject
+internal struct DictionaryBatch : IFlatbufferObject
 {
   private Table __p;
   public ByteBuffer ByteBuffer { get { return __p.bb; } }

src/Apache.Arrow/Flatbuf/DictionaryEncoding.cs

@@ -10,7 +10,7 @@ namespace Apache.Arrow.Flatbuf
 
 /// ----------------------------------------------------------------------
 /// Dictionary encoding metadata
-public struct DictionaryEncoding : IFlatbufferObject
+internal struct DictionaryEncoding : IFlatbufferObject
 {
   private Table __p;
   public ByteBuffer ByteBuffer { get { return __p.bb; } }

src/Apache.Arrow/Flatbuf/Enums/DateUnit.cs

@@ -5,7 +5,7 @@
 namespace Apache.Arrow.Flatbuf
 {
 
-public enum DateUnit : short
+internal enum DateUnit : short
 {
  DAY = 0,
  MILLISECOND = 1,

src/Apache.Arrow/Flatbuf/Enums/Endianness.cs

@@ -7,7 +7,7 @@ namespace Apache.Arrow.Flatbuf
 
 /// ----------------------------------------------------------------------
 /// Endianness of the platform producing the data
-public enum Endianness : short
+internal enum Endianness : short
 {
  Little = 0,
  Big = 1,

src/Apache.Arrow/Flatbuf/Enums/IntervalUnit.cs

@@ -5,7 +5,7 @@
 namespace Apache.Arrow.Flatbuf
 {
 
-public enum IntervalUnit : short
+internal enum IntervalUnit : short
 {
  YEAR_MONTH = 0,
  DAY_TIME = 1,

src/Apache.Arrow/Flatbuf/Enums/MessageHeader.cs

@@ -13,7 +13,7 @@ namespace Apache.Arrow.Flatbuf
 /// Arrow implementations do not need to implement all of the message types,
 /// which may include experimental metadata types. For maximum compatibility,
 /// it is best to send data using RecordBatch
-public enum MessageHeader : byte
+internal enum MessageHeader : byte
 {
  NONE = 0,
  Schema = 1,

src/Apache.Arrow/Flatbuf/Enums/MetadataVersion.cs

@@ -5,7 +5,7 @@
 namespace Apache.Arrow.Flatbuf
 {
 
-public enum MetadataVersion : short
+internal enum MetadataVersion : short
 {
   /// 0.1.0
  V1 = 0,