[DocDB] Use pg_txn_start_us for determining txn start time in deadlock detector

[robertsami]

Jira Link: DB-9952

Description

With the introduction of #14165, we will start resolving deadlocks based on txn start time. We currently report the txn start time based on client/transaction.cc start time to the txn coordinator. However, wait queues use pg_txn_start_us, computed by the pg session, to determine resumption order. We should use the same source of time in both places, and prefer to use pg's notion of start time

Issue Type

kind/bug

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