perf(stream): stateless 2-phase agg, state local hash agg

[st1page]

We have had a stateless 2-phase simple agg to prevent single-point bottlenecks in simple aggregation with less costs.

risingwave/src/frontend/src/optimizer/plan_node/logical_agg.rs

Lines 196 to 206 in 4292d9f

	// Stateless 2-phase simple agg
	// can be applied on stateless simple agg calls,
	// with input distributed by [`Distribution::AnyShard`]
	if self.group_key().is_empty()
	&& self
	.core
	.all_local_aggs_are_stateless(stream_input.append_only())
	&& input_dist.satisfies(&RequiredDist::AnyShard)
	{
	return self.gen_stateless_two_phase_streaming_agg_plan(stream_input);
	}

risingwave/src/stream/src/executor/stateless_simple_agg.rs

Line 29 in cd28ff0

pub struct StatelessSimpleAggExecutor {

But we now found when a hotspot key exists. that is found in NEXMark q15. https://risingwave-labs.slack.com/archives/C0543LNH0KU/p1699324490070239?thread_ts=1699323766.819059&cid=C0543LNH0KU.
The stateless 2-phase optmization is also needed.

[stdrc]

Any more information?

[st1page]

Any more information?

Yes I will add it.

[github-actions]

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