sql: use Clock.PhysicalTime in beginTransactionTimestampsAndReadMode

Synchronizing with the HLC clock doesn't look to be necessary. I'm confused
about this though. The comment on beginTransactionTimestampsAndReadMode says
that "txnSQLTimestamp propagates to become the TxnTimestamp". Is this trying to
say that the timestamp makes it way into the kv.Txn? Because that's not true.

Regardless, the one reason not to make this change is that PhysicalTime is not
guaranteed to be monotonic on some systems and can generally diverge from the
HLC's clock. If we're worried about that though, we should use the HLC here and
feed that directly into the kv.Txn. We shouldn't need to grab two timestamps
from the HLC per txn.

pkg/sql/conn_executor_exec.go

@@ -916,14 +916,14 @@ func (ex *connExecutor) beginTransactionTimestampsAndReadMode(
 	historicalTimestamp *hlc.Timestamp,
 	err error,
 ) {
-	now := ex.server.cfg.Clock.Now()
+	now := ex.server.cfg.Clock.PhysicalTime()
 	if s.Modes.AsOf.Expr == nil {
 		rwMode = ex.readWriteModeWithSessionDefault(s.Modes.ReadWriteMode)
-		return rwMode, now.GoTime(), nil, nil
+		return rwMode, now, nil, nil
 	}
 	ex.statsCollector.reset(&ex.server.sqlStats, ex.appStats, &ex.phaseTimes)
 	p := &ex.planner
-	ex.resetPlanner(ctx, p, nil /* txn */, now.GoTime())
+	ex.resetPlanner(ctx, p, nil /* txn */, now)
 	ts, err := p.EvalAsOfTimestamp(s.Modes.AsOf)
 	if err != nil {
 		return 0, time.Time{}, nil, err