Add the synchronized decorators before ports' critical sections

When multiple port updates occur we experience transaction timeouts.
This patch encloses `create_port` and `delete_port` to guarantee that
only one thread enters the critical section.

This fixes [MN-921][mn921], which is a bug for Havana. The patch for Grizzly is
already merged and [MN-908][mn908] is resolved.

Signed-off-by: Taku Fukushima <tfukushima@midokura.com>

[mn921]: https://midobugs.atlassian.net/browse/MN-921
[mn908]: https://midobugs.atlassian.net/browse/MN-908