Fix error recovery, cleanup session handles. Always reschedule task

Fix error recovery, cleanup session handles. Always reschedule task
at end of do_syncrepl; this fixes the problem of syncrepl never starting
if the master isn't up. Set sync interval to 60 seconds in persist mode;
it will retry connecting to the master when the master isn't up. Fail on
SASL at config time instead of at runtime when SASL is not enabled.