[bgpcfgd] bgp peer is not added successfully sometimes

[byu343]

Description
We experienced an random error on bgpcfgd:
Dec 9 20:07:44.264702 ck417 ERR bgp#bgpcfgd: Peer 10.0.0.1. Error in rendering the template for "SET" command {'rrclient': '0', 'name': 'router1', 'local_addr': '10.0.0.0', 'nhopself': '0', 'holdtime': '180', 'asn': '8074', 'keepalive': '60'}

The result of this error is bgp peer not being added.

The error should be raised from the line: https://github.com/Azure/sonic-buildimage/blob/d39f10b31fafeb4ca55d6551bfa1cba26e384306/dockers/docker-fpm-frr/bgpcfgd#L305.

This may be related to the logic under the line "new_bgp_messages.append((key, op, data))" in "a73eb66#diff-a1938483aedd0ca4c64191d18290bca9L136" being removed by a recent change.
Specifically, under the latest code, if the message of adding peer is handled earlier than the one for filling neighbor meta data, the adding peer action will fail, and not get reprocessed later.

Steps to reproduce the issue:

1. Running sonic for the first time on a device, where the initial configs including bgp settings are stored in /mnt/flash/minigraph.xml

Describe the results you received:
With certain probability, the bgp peer is not being added after booting, and in turn the bgp session will not be established.

Describe the results you expected:
The bgp peer is added after the first booting of sonic.

Additional information you deem important (e.g. issue happens only occasionally):

**Output of `show version`:**

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**Attach debug file `sudo generate_dump`:**

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