rpc: make OncRpcException subtype of IOException

Acked-by: Gerd Behrmann
Target: master
xdr: introduce BadXdrOncRpcException

the exception is thrown when xdr stream can't be decoded.
Remove redundant decoder code.

Acked-by: Gerd Behrmann
Target: master
xdr: introduce Xdr#hasMoreData()

Acked-by: Gerd Behrmann
Target: master