bridge: Wait for previous stats update transation when it is incomplete.

bridge: Wait for previous stats update transation when it is incomplete.

When ovsdb happens to get blocked for few seconds, the 'stats_timer'
will not be updated due to incompletion of previous transaction.
When the current time passes the 'stats_timer', the call to
poll_timer_wait_until(stats_timer) will keep waking up ovs-vswitchd,
causing 100% cpu utilization.

This commit fixes this issue by making ovs-vswitchd wait on the
previous idl transaction when it is incomplete rather than the
'stats_timer'.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>