Reconsider process info implementation for statement context reuse

[tiancaiamao]

Enhancement

To reduce object allocation in the point get, I aggressively reuse the objects.
When I try to reuse the StatementContext in #26007, I found some problem.

In the past, SetProcessInfo() get a snapshot of the process info:

	pi := util.ProcessInfo{
		ID:               s.sessionVars.ConnectionID,
		Port:             s.sessionVars.Port,
		DB:               s.sessionVars.CurrentDB,
		Command:          command,
		Plan:             p,
		PlanExplainRows:  plannercore.GetExplainRowsForPlan(p),
		RuntimeStatsColl: s.sessionVars.StmtCtx.RuntimeStatsColl,
		Time:             t,
		State:            s.Status(),
		Info:             sql,
		CurTxnStartTS:    curTxnStartTS,
		StmtCtx:          s.sessionVars.StmtCtx,
		StatsInfo:        plannercore.GetStatsInfo,
		MaxExecutionTime: maxExecutionTime,
		RedactSQL:        s.sessionVars.EnableRedactLog,
	}
	s.processInfo.Store(&pi)

This is a shallow copy, when ResetContextStmt() create objects, the pi referred objects are never touched again.
But if I reuse those object, the pi will be modified too! It's not a real snapshot any more.

So we can not reuse the old StmtCtx is the a new statement .... the pi is still referencing it.

[xhebox]

Hmm, I think the PR could be reopened after solving the issue. My first idea is to provide a Clone for StmtCtx, and remove RuntimeStatsColl in the processinfo.