There might be duplicated enum values when allow_alias is true. Add PreferredAlias into OriginalNameAttribute to remove the duplication (#2727)

This consists of:
- Changing the codegen for the fixed set of options protos, to parse unknown fields instead of skipping them
- Add a new CustomOptions type in the C# support library
- Expose CustomOptions properties from the immutable proto wrappers in the support library

Only single-value options are currently supported, and fetching options values requires getting the type right
and knowing the field number. Both of these can be addressed at a later time.

Fixes #2143, at least as a first pass.

* Factored Conformance test messages into shared test schema.

* Updated benchmarks to use new proto3 message locations.

* Fixed include path.

* Conformance: fixed include of Python test messages.

* Make maven in Rakefile use --batch-mode.

* Revert changes to benchmarks.

On second thought I think a separate schema for
CPU benchmarking makes sense.

* Try regenerating C# protos for new test protos.

* Removed benchmark messages from test proto.

* Added Jon Skeet's fixes for C#.

* Removed duplicate/old test messages C# file.

* C# fixes for test schema move.

* Fixed C# to use the correct TestAllTypes message.

* Fixes for Objective C test schema move.

* Added missing EXTRA_DIST file.

JSON tests fail, as we're not using OriginalNameAttribute yet.

Conflicts:
	csharp/src/Google.Protobuf/JsonFormatter.cs

"valueField": null

is parsed appropriately, i.e. that it remembers that the field is set.

This fixes issue #933, effectively.

On deserialization, missing values for message types
are replaced with a "default" message.

Generated code changes for previous commit (basically InternalBuildGeneratedFileFrom => FromGeneratedCode)

Other changes are due to the well-known types changing without us regenerating.

We'll see what I've missed when CI fails...