Summary:
SetPgTxnStart is invoked from SetupSession, but SetupSession could be called multiple time while transaction is running.
It results in data race accessing pg_txn_start_us in metadata.

Moved call to SetPgTxnStart to DoBeginTransactionIfNecessary, which invoked only once per transaction.
Jira: DB-13619

Test Plan: ./yb_build.sh tsan --cxx-test pg_tablet_split-test --gtest_filter PgRangePartitionedTableSplitTest.SelectMiddleRangeAfterManualSplit -n 120

Reviewers: pjain, bkolagani

Reviewed By: bkolagani

Subscribers: ybase, yql

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D39339