ofproto-dpif.at: Wait for the monitor's pidfile disappears where necessary

These tests invokes ovs-ofctl monitor twice or more.
Because "ovs-appctl -t ofctl exit" does not wait for the target
process exit, there are chances to see the pid file from the previous
incarnation.

OVS_APP_EXIT_AND_WAIT macro was provided by Ben Pfaff.

Acked-by: Ben Pfaff <blp@nicira.com>
Co-authored-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
ovsdb-server.at: Update a comment

Acked-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>