Ensure cached query results are discarded

[linzebing]

Description

Is this change a fix, improvement, new feature, refactoring, or other?

Fix.

Previous attempt was da2cd0a, but it doesn't work out in the following cases:

1. If query only needs to call getNextResult once, on the first call lastResult is null, we will not have the next token to clear the cached result
2. It's possible for exchange to return empty result yet not finished. In this case we will not have the next token to clear the cached result either.

This PR improves the logic to check resultRows, as lastResult will be set to a QueryResults containing resultRows at the end. If it's non-empty, then we need next token to clear the cached result. Note that the queryInfo.getOutputStage().isPresent() check is needed as

trino/core/trino-main/src/main/java/io/trino/server/protocol/Query.java

Lines 510 to 515 in 736c160

	// For queries with no output, return a fake boolean result for clients that require it.
	if ((queryInfo.getState() == QueryState.FINISHED) && queryInfo.getOutputStage().isEmpty()) {
	return queryResultRowsBuilder(session)
	.withSingleBooleanValue(createColumn("result", BooleanType.BOOLEAN), true)
	.build();
	}

Is this a change to the core query engine, a connector, client library, or the SPI interfaces? (be specific)

Core engine.

How would you describe this change to a non-technical end user or system administrator?

This ensures cached query results are discarded upon completion, which improves memory usage of coordinator.

Related issues, pull requests, and links

Documentation

(x) No documentation is needed.
( ) Sufficient documentation is included in this PR.
( ) Documentation PR is available with #prnumber.
( ) Documentation issue #issuenumber is filed, and can be handled later.

Release notes

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( ) Release notes entries required with the following suggested text:

# Section
* Fix some things. ({issue}`issuenumber`)