Support upsert mode pg -> pg

[iskakaushik]

fixes: #553

[saisrirampur]

@iskakaushik first run is erroring out with:

{
"message": "failed to sync records: failed to create staging table: ERROR: syntax error at or near "[" (SQLSTATE 42601)",
"source": "GoSDK",
"stackTrace": "",
"encodedAttributes": null,
"cause": {
"message": "failed to create staging table: ERROR: syntax error at or near "[" (SQLSTATE 42601)",
"source": "GoSDK",
"stackTrace": "",
"encodedAttributes": null,
"cause": null,
"applicationFailureInfo": {
"type": "",
"nonRetryable": false,
"details": null
}
},
"applicationFailureInfo": {
"type": "wrapError",
"nonRetryable": false,
"details": null
}
}