speed startup: immediately run first monitor event after startup.

speed startup: immediately run first monitor event after startup.

Once we've done a startup, we need to run a monitor event successfully
to be marked as healthy.  Rather than wait the usual 5 seconds, run it
immediately (which will then reset next_interval to 5 seconds).

Seconds between ctdbd first log message and node healthy:
BEFORE:	23.58
AFTER: 18.09

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>