Parser-NG: Convert the ConnStateData input buffer to SBuf

Prepare the way to efficiently parse client requests using SBuf based
parser-ng.

IoCallback stores a raw-pointer to the ConnStateData::In::buf member
object rather than an SBuf reference to the backing MemBlob or char*
store so that only the short (blocking) FD_READ_METHOD() call needs to
provide any synchronous guarantees. We also particularly need a direct
(raw) pointer to the ConnStateData member to prevent the possible
read/consume collisions causing problems with the ConnStateData callback
and avoid having to merge two separate SBuf.