Change stream I/O behaviour around sockets

* Previously each read and write would 1:1 become a send or recv call.
* Now we buffer writes and send in batch (or when a chunk finishes),
  and every time we read we try to non-blocking read more data to fill
  the buffer, to allow batching reads where possible without blocking
  on data that will never come.

renderdoc/serialise/serialiser.cpp

@@ -404,6 +404,8 @@ void Serialiser<SerialiserMode::Writing>::EndChunk()
   m_Write->AlignTo<ChunkAlignment>();
 
   m_ChunkMetadata = SDChunkMetaData();
+
+  m_Write->Flush();
 }
 
 template <>