So that it doesn't have to scan over the request buffer again on reply. This also allow us to store additional info like memory mapping
Right now only MappedBuffers that only span a single page and are not aligned are implemented. MappedBuffers are unmapped during the reply part of ReplyAndReceive. Only unmapping of ReadOnly buffers is currently implemented.
Currently only CopyHandle, MoveHandle and CallingPid descriptors are implemented.