Push SBuf down into Comm::IoCallback

Push SBuf down into Comm::IoCallback

Use a pointer to the SBuf instead of the rawSpace() because using a SBuf
copy can potentially cause different MemBlob to exist behind the Comm
read classes and the caller (ConnStateData) class. They need to be kept
identical so as not to loose any existing bytes in the I/O buffer when
a read callback handler is run.

Pointer to SBuf also avoids race conditions between comm_read async calls
and BodyPipe notifications which can potentially change the MemBlob
underneath comm_read() and invalidate te char* buffer pointer used
previously.