[GCU] Optimize full key deletion/addition, to be done in a single step rather than multiple

[ghooo]

Description

Example:

admin@vlab-01:~/everflow$ cat add_session.json2
[ {
  "op": "add",
  "path": "/MIRROR_SESSION",
  "value": {
   "mirror_session_dscp": {
    "dscp": "5",
    "dst_ip": "2.2.2.2",
    "src_ip": "1.1.1.1",
    "ttl": "32",
    "type": "ERSPAN"
   }
  }
 }
 ]
admin@vlab-01:~/everflow$ sudo config apply-patch add_session.json2
Patch Applier: Patch application starting.
Patch Applier: Patch: [{"op": "add", "path": "/MIRROR_SESSION", "value": {"mirror_session_dscp": {"dscp": "5", "dst_ip": "2.2.2.2", "src_ip": "1.1.1.1", "ttl": "32", "type": "ERSPAN"}}}]
Patch Applier: Getting current config db.
Patch Applier: Simulating the target full config after applying the patch.
Patch Applier: Validating target config does not have empty tables, since they do not show up in ConfigDb.
...
Patch Applier: Applying 5 changes in order:
Patch Applier:   * [{"op": "add", "path": "/MIRROR_SESSION", "value": {"mirror_session_dscp": {"dscp": "5"}}}]
Patch Applier:   * [{"op": "add", "path": "/MIRROR_SESSION/mirror_session_dscp/dst_ip", "value": "2.2.2.2"}]
Patch Applier:   * [{"op": "add", "path": "/MIRROR_SESSION/mirror_session_dscp/src_ip", "value": "1.1.1.1"}]
Patch Applier:   * [{"op": "add", "path": "/MIRROR_SESSION/mirror_session_dscp/ttl", "value": "32"}]
Patch Applier:   * [{"op": "add", "path": "/MIRROR_SESSION/mirror_session_dscp/type", "value": "ERSPAN"}]
Patch Applier: Verifying patch updates are reflected on ConfigDB.
Patch Applier: Patch application completed.
Patch applied successfully.

The generated steps are 5 changes. This can be optimized to only 1 step.
i.e.

Patch Applier: Applying 1 changes in order:
Patch Applier:   * [{"op": "add", "path": "/MIRROR_SESSION", "value": {"mirror_session_dscp": {"dscp": "5", "dst_ip": "2.2.2.2", "src_ip": "1.1.1.1", "ttl": "32", "type": "ERSPAN"}}}]
Patch Applier: Verifying patch updates are reflected on ConfigDB.
Patch Applier: Patch application completed.
Patch applied successfully.

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