rconn: Avoid occasional immediate connection failures.

rconn: Avoid occasional immediate connection failures.

The rconn connection timer measures time on the granularity of seconds,
which means that sometimes the actual timeout can be just a millsecond or
so, which led to occasional immediate connection failures from ovs-ofctl.

VMware-BZ: #2295760
Fixes: 476d2551abd2 ("rconn: Introduce new invariant to fix assertion failure in corner case.")
Reported-by: Ken Ajiro <ken-ajiro@xr.jp.nec.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>