[NFS GW] Fix regression after netty4 migration

Two bugs were found recently. Both related to hadoop portmap utility.

(1) The ChannelHandler that can be added to a ChannelPipeline in Netty4 must be Shareable. However, IdleStateHandler, used by Portmap is not. We need to fix it otherwise hadoop portmap may fail to start.

(2) RpcUdpResponseStage does not get the request from client (hadoop mountd) because of response type mismatch, and therefore it does not respond back to client request.

This is a regression from HADOOP-11245 but I consider it not a blocker, because most recent Linux distro supports rpcbind and there's little reason to use Hadoop's portmap.