entity structures are kept even when the entity is removed. can be used as DOS attack

when a node connects to ovsdb southbound, there is an entity-owner node
created. for single node, it looks as below. However, when the device
disconnects, it still remains. It does correctly reflect that there is no
ownershipt, but it should be removed entirely.

when node connected:

"entity-owners": {
"entity-type": [
{
"entity": [
{
"candidate": [

{ "name": "member-1" }

],
"id": "/network-topology:network-topology/network-topology:topology[network-topology:topology-id='ovsdb:1']/network-topology:node[network-topology:node-id='ovsdb://uuid/bc330598-4581-4d9f-b932-e362b452137b']",
"owner": "member-1"
}
],
"type": "ovsdb"
},
{
"entity": [
{
"candidate": [

{ "name": "member-1" }

],
"id": "/general-entity:entity[general-entity:name='ovsdb-southbound-provider']",
"owner": "member-1"
}
],
"type": "ovsdb-southbound-provider"
}
]
}
}

when node disconnected:

"entity-owners": {
"entity-type": [
{
"entity": [

{ "id": "/network-topology:network-topology/network-topology:topology[network-topology:topology-id='ovsdb:1']/network-topology:node[network-topology:node-id='ovsdb://uuid/bc330598-4581-4d9f-b932-e362b452137b']", "owner": "" }

],
"type": "ovsdb"
},
{
"entity": [
{
"candidate": [

{ "name": "member-1" }

],
"id": "/general-entity:entity[general-entity:name='ovsdb-southbound-provider']",
"owner": "member-1"
}
],
"type": "ovsdb-southbound-provider"
}
]
}
}