northd: Add northd change handler for sync_to_sb_lb node.

Any changes to northd engine node due to load balancers
are now handled in 'sync_to_sb_lb' node to sync the changed
load balancers to SB load balancers.

The logic to sync the SB load balancers is changed a bit and it
now mimics the SB lflow sync.

Below are the scale testing results done with all the patches applied
in this series using ovn-heater.  The test ran the scenario  -
ocp-500-density-heavy.yml [1].

The resuts are:

-------------------------------------------------------------------------------------------------------------------------------------------------------
			Min (s)		Median (s)	90%ile (s)	99%ile (s)	Max (s)		Mean (s)	Total (s)	Count	Failed
-------------------------------------------------------------------------------------------------------------------------------------------------------
Iteration Total		0.136883	1.129016	1.192001	1.204167	1.212728	0.665017	83.127099	125	0
Namespace.add_ports	0.005216	0.005736	0.007034	0.015486	0.018978	0.006211	0.776373	125	0
WorkerNode.bind_port	0.035030	0.046082	0.052469	0.058293	0.060311	0.045973	11.493259	250	0
WorkerNode.ping_port	0.005057	0.006727	1.047692	1.069253	1.071336	0.266896	66.724094	250	0
-------------------------------------------------------------------------------------------------------------------------------------------------------

The results with the present main [2] are:

-------------------------------------------------------------------------------------------------------------------------------------------------------
			Min (s)		Median (s)	90%ile (s)	99%ile (s)	Max (s)		Mean (s)	Total (s)	Count	Failed
-------------------------------------------------------------------------------------------------------------------------------------------------------
Iteration Total		0.135491	2.223805	3.311270	3.339078	3.345346	1.729172	216.146495	125	0
Namespace.add_ports	0.005380	0.005744	0.006819	0.018773	0.020800	0.006292	0.786532	125	0
WorkerNode.bind_port	0.034179	0.046055	0.053488	0.058801	0.071043	0.046117	11.529311	250	0
WorkerNode.ping_port	0.004956	0.006952	3.086952	3.191743	3.192807	0.791544	197.886026	250	0
-------------------------------------------------------------------------------------------------------------------------------------------------------

[1] - https://github.com/ovn-org/ovn-heater/blob/main/test-scenarios/ocp-500-density-heavy.yml
[2] - 2a12cda("controller, northd: Wait for cleanup before replying to exit")

Signed-off-by: Numan Siddique <[email protected]>