Allow list of services in metadata

Summary:
I'm changing the semantics of `metadata.thrift` here slightly.
 ---
Before:
- `ThriftServiceContext` contains `ThriftService` inline.
- `ThriftMetadata` only contains other services *referred* to by the primary service (in `ThriftServiceContext`). This includes all base classes **but not the service itself**.
- Understanding the service class hierarchy requires traversing through each `parent` field and looking up the name in `ThriftMetadata`.
 ---
After:
- `ThriftServiceContextRef` contains just the service name.
- `ThriftMetadata` now includes the service itself (previously inlined in `ThriftServiceContext`).
- `services` field lists all names of services in the class hierarchy in order from most to least derived. **These semantics are needed to support `MultiplexAsyncProcessorFactory` where the concept of a single `parent` falls apart**.
 ---

After migrating all clients, we can remove `ThriftServiceContext` completely. It's now deprecated.

For `py3`, I've removed `extractMetadataFromServiceContext` because it's no longer needed. All it was doing was adding the inline `ThriftServiceContext` into the metadata's map... which we do by default now.

Reviewed By: yfeldblum

Differential Revision: D29952004

fbshipit-source-id: 13c62aafabbfc287ad64489c02104dd977be71ce

eden/mononoke/scs_server/metadata.cpp

@@ -26,15 +26,11 @@ namespace scm {
 namespace service {
 
 rust::srserver::RustThriftMetadata* create_metadata() noexcept {
-  using apache::thrift::can_throw;
-
   auto meta = new rust::srserver::RustThriftMetadata();
   meta->meta_ =
       [](apache::thrift::metadata::ThriftServiceMetadataResponse& response) {
         ::apache::thrift::detail::md::ServiceMetadata<
-            ::facebook::scm::service::SourceControlServiceSvIf>::
-            gen(can_throw(*response.metadata_ref()),
-                can_throw(*response.context_ref()));
+            ::facebook::scm::service::SourceControlServiceSvIf>::gen(response);
       };
   return meta;
 }