bgp error handling changes for review on master branch (#424)

* bgp error handling changes for review on master branch

* changes for supporting positive ACK for BGP error handling

* Updated with changes for config replay

* Updated with changes for config replay

* Updated warm-reboot (section 6) and config knob(3.7.2)

doc/bgp_error_handling/images/bgp_error_handling_flow2.xml

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doc/bgp_error_handling/images/bgp_error_handling_flow3.xml

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doc/bgp_error_handling/images/bgp_error_handling_flow4.xml

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