Since we already disable job notification with +m, it doesn't
make sense to set it in the original invocation!

It's also annoying if all of the host commands end up overwriting
your regular bash_history!!

Batch startup support for OVS

Currently, every ovs-vsctl command requires reading the entire OVS
configuration database. This means that its performance gets linearly
slower as more switches and ports are added. To mitigate this, we
batch multiple configuration operations into individual, long,
ovs-vsctl commands.

This patch set makes a couple of other notable changes, including
setting printPid=False by default (avoids using mnexec unnecessarily)
and running certain commands using errRun rather than quietRun.
Additionally we no longer look for leftover links in the root
namespace, so code relying on that functionality may have to change
slightly (as in controlnet.py and sshd.py for example.) It also adds
cluster support to mn -c.

The performance result is that mn --topo linear,200 --test none now
completes in 60 seconds rather than 95 seconds (on my laptop) without
the patch (vs. 101 seconds in 2.2.0).

This is still slower than I would like - we should be able to make
some additional improvements.

If you try to use the low-level API, you are probably
not going to call batchStartup()! So, we set batch=False
by default. This means that buildFromTopo() needs to set
it to True, so we add a bit of irritatingly complex machinery
to allow this to happen. The good fallout of this is that
now customConstructor() returns a real subclass, not simply
a constructor function! We also detect errors where people
are incorrectly attempting to give parameters to a lambda
function - since none of our lambdas accept parameters!!

Note that this is a bit like functools.partial for classes -
it would be nice if functools had a true subclassing function.

Note that we are changing the interface of batchStartup/Shutdown
slightly so that the method can choose not to start some of the
switches. We might wish to refine this a bit...

This should rarely happen - in the usual case, either the
links will be shut down by Mininet.stop(), or the interfaces
will be deleted by node.stop( deleteIntfs=True ), or the
links or interfaces will be explicitly deleted or stopped
using the low-level API.

Cases that are relying on links being automatically deleted
in cleanup() will potentially find that they are now no longer
deleted, but these cases should be rare.

We still need to set it in batchShutdown()

This implements batch startup for OVS switches.

In the future, we would like to support higher bandwidth
limits for TCIntfs, but we'll probably need to adjust some
of the parameters dynamically.

In my opinion, we really shouldn't even get POLLHUP, but
we do. In case we get anything else odd, quite polling on
that fd.

It's tricky to get this right, but basically we want to read
if there is something to read; if not, we want to check for
EOF.

Without this, we end up retrying until we fail, instead
of returning immediately on success!

We should think a bit about the semantics that we want here.
The comments say "stop and clean up link" so perhaps that's
what we want. However, we could also imagine stop stopping
forwarding on the link (and possibly allowing restarts).

We warn on exited node.cmd() because we terminate the controller
before stopping/deleting the links. This makes sense to avoid a
storm of link/port down events, but since the controller's
shell has exited we cannot call link.stop() on any of its
links. We may want to simply stop the controller and not
terminate it, but at least it doesn't hang for now.

This allows --switch ovsbr,stp=True to work correctly

With newer versions of OVS, this allows us to set up a switch
with a single OVS command (if reconnectms is zero.) If reconnectms
is specified, then it slows things down slightly (but not much.)

Eventually we should implement true batch shutdown.
In the mean time, we just ignore it. Note there's no good
way that I know of for a subclass to remove a superclass
method, so we changed the protocol a bit to require a return
value of True.