Proxy side net_write_timeout and net_read_timeout

Put a proxy(mxs) between client and db will let server variables net_write_timeout and net_read_timeout not work， for example，if current client is hang，server's reponse cannot send to client, it will block on tcp send operation, with time gos on, it will trigger net_write_timeout to break this conneciion, but if put proxy(mxs) in the middle, if proxy is health ,server can always send data to proxy without block;

Proposal:
Use session track mechanism to track latest net_write_timeout/net_read_timeout value, and apply it in proxy side;