Make processSerializedRequest non pure virtual
Make processSerializedRequest non pure virtual

Summary: To start removing processSerializedRequest call sites (see [this_quip](https://fb.quip.com/cqP7Ap4UPLY8) for context), we need to make in non-pure virtual.

Reviewed By: ycking21

Differential Revision: D37796530

fbshipit-source-id: 9ab72a10cf2425e5e53667d1d98046b8d3073208

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift