[#24778] DocDB: Set field pg_txn_start_us for restarted txns

Summary:
Commit 269a518 / D39339 rightly changed the way we set field `pg_txn_start_us` for docdb transactions, by setting it during the first call to `DoBeginTransactionIfNecessary` instead of setting it on each `Perform` call.

We seem to have missed setting the field when a new transaction is created through `PgClientSession::RestartTransaction`. This path is triggered by transactions that encounter read restart error, and that undergo retries triggered by the query layer.

`pg_txn_start_us` field is used by the wait-queue to resume waiters in order of earliest transactions first i.e for a set of resolved waiters, the resumption is in the order - earliest to the latest txn start time. The wait-queue has a DCHECK ensuring that docdb distributed txns have a non-zero txn start time, which was not being set for txns restarted due to read restarts, and hence the failures of test `TestPgTransparentRestarts#testUpdateLong`.

This diff addresses the issue by explicitly copying `pg_txn_start_us` in `YBTransaction::Impl::FillRestartedTransaction`, essentially restoring the behavior of preserving priority of waiter resumption for read-restarted txns.
Jira: DB-13873

Test Plan:
Jenkins

./yb_build.sh fastdebug --java-test TestPgTransparentRestarts#testUpdateLong -n 10 --tp 1

Reviewers: sergei, pjain, patnaik.balivada

Reviewed By: sergei

Subscribers: ybase

Differential Revision: https://phorge.dev.yugabyte.com/D39713

src/yb/client/transaction.cc

@@ -394,6 +394,7 @@ class YBTransaction::Impl final : public internal::TxnBatcherIf {
       } else {
         other->metadata_.start_time = other->read_point_.Now();
       }
+      other->metadata_.pg_txn_start_us = metadata_.pg_txn_start_us;
       state_.store(TransactionState::kAborted, std::memory_order_release);
     }
     DoAbort(TransactionRpcDeadline(), transaction);

src/yb/docdb/wait_queue.cc

@@ -277,7 +277,7 @@ struct WaiterData : public std::enable_shared_from_this<WaiterData> {
         finished_waiting_latency_(*finished_waiting_latency),
         unlocked_(locks->Unlock()),
         deadline_(deadline) {
-    DCHECK(txn_start_us || id.IsNil());
+    LOG_IF_WITH_PREFIX(DFATAL, !txn_start_us && !id.IsNil()) << "Expected non-zero txn_start_us";
     VLOG_WITH_PREFIX(4) << "Constructed waiter";
   }