dpif-netdev: Centralized threads and queues handling code.

Currently we have three different code paths that deal with pmd threads
and queues, in response to different input

1. When a port is added
2. When a port is deleted
3. When the cpumask changes or a port must be reconfigured.

1. and 2. are carefully written to minimize disruption to the running
datapath, while 3. brings down all the threads reconfigure all the ports
and restarts everything.

This commit removes the three separate code paths by introducing the
reconfigure_datapath() function, that takes care of adapting the pmd
threads and queues to the current datapath configuration, no matter how
we got there.

This aims at simplifying maintenance and introduces a long overdue
improvement: port reconfiguration (can happen quite frequently for
dpdkvhost ports) is now done without shutting down the whole datapath,
but just by temporarily removing the port that needs to be reconfigured
(while the rest of the datapath is running).

We now also recompute the rxq scheduling from scratch every time a port
is added of deleted.  This means that the queues will be more balanced,
especially when dealing with explicit rxq-affinity from the user
(without shutting down the threads and restarting them), but it also
means that adding or deleting a port might cause existing queues to be
moved between pmd threads.  This negative effect can be avoided by
taking into account the existing distribution when computing the new
scheduling, but I considered code clarity and fast reconfiguration more
important than optimizing port addition or removal (a port is added and
removed only once, but can be reconfigured many times)

Lastly, this commit moves the pmd threads state away from ovs-numa.  Now
the pmd threads state is kept only in dpif-netdev.

Signed-off-by: Daniele Di Proietto <[email protected]>
Co-authored-by: Ilya Maximets <[email protected]>
Signed-off-by: Ilya Maximets <[email protected]>
Acked-by: Ilya Maximets <[email protected]>