Simplified MemObject::write() API.

Simplified MemObject::write() API.

The API required a callback, but the call was always synchronous and the
required callback mechanism could not reliably support an async call anyway.

The method adjusted the buffer offset to become relative to headers rather
than body. While the intent to separate headers from body is noble, none of
the existing caches support that separation, and a different API will be
needed to support it correctly anyway. For now, let's reduce the number of
special cases and offset manipulations.