Update third_party/protobuf to protobuf-v3.0.0-beta-3

third_party/protobuf/csharp/README.md

 This directory contains the C# Protocol Buffers runtime library.
 
-Status: Alpha - ready for early adopters
-========================================
-
-This code is still under significant churn. Unlike the original port,
-it only supports proto3 (but not *all* of proto3 yet) - there are no
-unknown fields or extensions, for example. protoc will (eventually)
-deliberately fail if it is asked to generate C# code for proto2
-messages other than descriptor.proto, which is still required for
-reflection. (It's currently exposed publicly, but won't be
-eventually.)
-
-Also unlike the original port, the new version embraces mutability -
-there are no builder types.
+Status: Beta - ready for external testing
+=========================================
 
 Usage
 =====
 
 The easiest way how to use C# protobufs is via the `Google.Protobuf`
 NuGet package. Just add the NuGet package to your VS project.
 
 Besides C# runtime library, the NuGet package also contains 
 precompiled version of `protoc.exe` and a copy of well known `.proto`
 files under the package's `tools` directory.
 
 To generate C# files from your `.proto` files, invoke `protoc` with the 
 `--csharp_out` option.
 
 Supported platforms
 ===================
 
 The runtime library is built as a portable class library, supporting:
 
 - .NET 4.5
 - Windows 8
 - Windows Phone Silverlight 8
 - Windows Phone 8.1
-- .NET Core (dnxcore)
+- .NET Core
+
+You should be able to use Protocol Buffers in Visual Studio 2012 and
+all later versions. This includes all code generated by `protoc`,
+which only uses features from C# 3 and earlier.
 
 Building
 ========
 
-Open the `src/Google.Protobuf.sln` solution in Visual Studio. Click "Build solution" to build the solution. You should be able to run the NUnit test from Test Explorer (you might need to install NUnit Visual Studio add-in).
+Open the `src/Google.Protobuf.sln` solution in Visual Studio 2015 or
+later. You should be able to run the NUnit test from Test Explorer
+(you might need to install NUnit Visual Studio add-in).
 
-Supported Visual Studio versions are VS2013 (update 4) and VS2015. On Linux, you can also use Monodevelop 5.9 (older versions might work fine).
+Although *users* of this project are only expected to have Visual
+Studio 2012 or later, *developers* of the library are required to
+have Visual Studio 2015 or later, as the library uses C# 6 features
+in its implementation. These features have no impact when using the
+compiled code - they're only relevant when building the
+`Google.Protobuf` assembly.
 
 History of C# protobufs
 =======================
 
 This subtree was originally imported from https://github.com/jskeet/protobuf-csharp-port
 and represents the latest development version of C# protobufs, that will now be developed
 and maintained by Google. All the development will be done in open, under this repository
 (https://github.com/google/protobuf).
+
+The previous project differs from this project in a number of ways:
+
+- The old code only supported proto2; the new code only supports
+proto3 (so no unknown fields, no required/optional distinction, no
+extensions)
+- The old code was based on immutable message types and builders for
+them
+- The old code did not support maps or `oneof`
+- The old code had its own JSON representation, whereas the new code
+uses the standard protobuf JSON representation
+- The old code had no notion of the "well-known types" which have
+special support in the new code
+- The old project supported some older platforms (such as older
+versions of Silverlight) which are not currently supported in the
+new project