[YSQL] Incomplete ErrorData sent from the background workers

[andrei-mart]

Jira Link: DB-11257

Description

When executing parallel query background workers send their errors and notices to the main process to forward to the client.
Send and receive functions used for that are the part of the Postgres client/server communication protocol implementation.
These functions do not transfer the yb_txn_errcode field.

This field is not forwarded to the client, but is used internally to transparently retry queries in case of statement/transaction conflicts.
Therefore parallel queries are not transparently restarted when statement/transaction conflict errors are coming to the background workers. Lost yb_txn_errcode value does not allow main process to properly identify the conflict.

Issue Type

kind/bug

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