[201911] add checking the connection between zebra and bgp

[lipxu]

Why I did it

Back port #6478 and #6519 to 201911 branch.

Work item tracking

• Microsoft ADO (number only):
  24978836

How I did it

Add checking the connection between zebra and bgp during bgpd start.

How to verify it

Modify start.h, add debug log and check the syslog

  _Sep 22 02:41:29.716356 str-a7060cx-acs-10 INFO bgp#root: ####: start zebra
  Sep 22 02:41:30.815341 str-a7060cx-acs-10 INFO bgp#root: ####: start check connection
  Sep 22 02:41:30.868784 str-a7060cx-acs-10 INFO bgp#root: ####: It took 0.029979 seconds to wait for zebra to be ready to accept connections
  Sep 22 02:41:30.873685 str-a7060cx-acs-10 INFO bgp#root: ####: start bgpd
  Sep 22 02:41:35.270569 str-a7060cx-acs-10 INFO bgp#root: ####: done_

  _Sep 22 03:28:02.423438 str-a7060cx-acs-10 INFO bgp#root: ####: start zebra
  Sep 22 03:28:03.731320 str-a7060cx-acs-10 INFO bgp#root: ####: start check connection
  Sep 22 03:28:33.749152 str-a7060cx-acs-10 INFO bgp#root: ####: Error: zebra is not ready to accept connections
  Sep 22 03:28:33.752490 str-a7060cx-acs-10 INFO bgp#root: ####: start bgpd
  Sep 22 03:28:34.259735 str-a7060cx-acs-10 INFO bgp#root: ####: start bgpd done
  Sep 22 03:28:34.755538 str-a7060cx-acs-10 INFO bgp#root: ####: start bgpcfgd
  Sep 22 03:28:35.800906 str-a7060cx-acs-10 INFO bgp#root: ####: done_

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111
• 202205
• 202211
• 202305

Tested branch (Please provide the tested image version)

Description for the changelog

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)

[StormLiangMS]

LGTM

[StormLiangMS]

@lipxu could you run a full nightly with this change?