ovsdb-server: Don't log closing session at program termination.

ovsdb-server: Don't log closing session at program termination.

When ovsdb-server closes a remote connection, it logs a message about it
that includes the reason.  Until now this has included sessions that it
closes when it exits.  That meant that, when --run was used, there was a
race between noticing that the subprocess exited and noticing that the
session that that subprocess (presumably) had open had been closed.  If
it noticed the latter first, nothing was logged (because it didn't log
anything if a session was closed in the ordinary way by the client).  If
it noticed the former first, it logged a message about closing the session
itself.

This is a benign race that causes no real problems--except that the tests
didn't expect to see the log message from the former case and fail with
errors like the following:

    1826. ovsdb-server.at:92: testing truncating database log with bad transaction ...
    ./ovsdb-server.at:96: ovsdb-tool create db schema
    stderr:
    stdout:
    ./ovsdb-server.at:104: ovsdb-server --remote=punix:socket db --run="sh txnfile"
    --- /dev/null   2018-04-24 08:50:58.769000000 +0000
    +++ /root/openvswitch-2.9.2/rpm/rpmbuild/BUILD/openvswitch-2.9.2/tests/testsuite.dir/at-groups/1826/stderr      2018-05-29 14:29:56.529257295 +0000
    @@ -0,0 +1,2 @@
    +2018-05-29T14:29:56Z|00001|ovsdb_jsonrpc_server|INFO|unix#0: disconnecting (removing ordinals database due to server termination)
    +2018-05-29T14:29:56Z|00002|ovsdb_jsonrpc_server|INFO|unix#0: disconnecting (removing _Server database due to server termination)

This fixes the race.  This particular log message isn't too useful since
it's pretty obvious that ovsdb-server is closing those sessions, since
after all it's exiting!

Reported-by: Sanket Sudake <sanket@infracloud.io>
Reported-at: https://mail.openvswitch.org/pipermail/ovs-discuss/2018-May/046840.html
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Numan Siddique <nusiddiq@redhat.com>