Use IOBufPtr for binary payload fields

Summary:
The bare "binary" type in Thrift IDL maps to std::string in the generated C++
code. On the deserialization path this hits the std::string overload of
apache::thrift::detail::readStringBody (Protocol.h:435), which calls
folly::io::Cursor::pullAtMost into the string's storage - a memcpy of the
full payload out of the receive IOBuf chain.

Switch every bare binary payload field in StressTest.thrift to the existing
IOBufPtr typedef, which is annotated with
`cpp.Type{name = "std::unique_ptr<folly::IOBuf>"}`. The generated reader then
dispatches to the IOBuf overload of readBinary, which clones buffer references
via Cursor::clone - refcount bumps, no payload memcpy.

This is a wire-compatible change (still serialized as a binary field) but is a
source-incompatible C++ API change: handlers/clients now see
`std::unique_ptr<folly::IOBuf>` instead of std::string. The stoptls_payload
fields already used IOBufPtr, so handler code already knows how to consume
the IOBuf form.

Differential Revision: D101410011

fbshipit-source-id: 94c337e83b83584acbee37fc31f875276058e62e

Author: spikeh

thrift/conformance/stresstest/if/StressTest.thrift

@@ -95,11 +95,11 @@ struct StorageReadRequest {
 }
 
 struct StorageReadResponse {
-  1: binary payload;
+  1: IOBufPtr payload;
 }
 
 struct StorageWriteRequest {
-  1: binary payload;
+  1: IOBufPtr payload;
 }
 
 struct StorageWriteResponse {}

thrift/conformance/stresstest/server/StressTestHandler.cpp

@@ -305,7 +305,10 @@ void StressTestHandler::async_tm_storageReadTm(
   auto response = std::make_unique<StorageReadResponse>();
   auto size = *request->responseBytes();
   if (size > 0) {
-    response->payload() = std::string(size, 'x');
+    auto buf = folly::IOBuf::create(size);
+    std::memset(buf->writableData(), 'x', size);
+    buf->append(size);
+    response->payload() = std::move(buf);
   }
   callback->result(std::move(response));
 }
@@ -316,7 +319,10 @@ void StressTestHandler::async_eb_storageReadEb(
   auto response = std::make_unique<StorageReadResponse>();
   auto size = *request->responseBytes();
   if (size > 0) {
-    response->payload() = std::string(size, 'x');
+    auto buf = folly::IOBuf::create(size);
+    std::memset(buf->writableData(), 'x', size);
+    buf->append(size);
+    response->payload() = std::move(buf);
   }
   callback->result(std::move(response));
 }