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, which is a bug for Havana. The patch for Grizzly is
already merged and MN-908 is resolved.

Signed-off-by: Taku Fukushima <tfukushima@midokura.com>