luxi: close and reopen the socket on errors

This is less of an actual issue for regular gnt-* clients, but it's
easily reproducible with burnin and possible with RAPI (depending on how
the program uses luxi.Client(s)).

In case of burnin, if we interrupt the client (^C) while it polls the
job, it will abort and raise an error. After that, burnin issues a
remove instance job, and at this point, we send the submit job (remove)
call but the first thing we read from the socket will be the response to
the previous poll job request, since that was queued already from the
master.

To solve this, whenever we detect an error in Transport.Call(), we close
that transport and re-create a new one, to start anew. The other
alternative would be to introduce a sequence to the protocol, but this
is something that would be design-level change and it's not recommended
at this stage.

Reviewed-by: imsnah