QueryFuzzer: replace element randomly when AST part buffer is full

[tomershafir]

Addressing a TODO:

 * TODO: we just stop remembering new parts after our corpus reaches certain size.
 * This is boring, should implement a random replacement of existing parst with
 * small probability. Do this after we add this fuzzer to CI and fix all the
 * problems it can routinely find even in this boring version.

I did a simple series of regression testing roughly comparing results of the following between old and new versions:

./build/programs/clickhouse client --query-fuzzer-runs 100 -q "SELECT (val + 257, (((tuple(NULL), 10.000100135803223), tuple(-inf)), '-1', (NULL, '0.10', NULL), NULL), (val + 9223372036854775807) = (rval * 100), tuple(65535), tuple(NULL), NULL, NULL), * FROM ( SELECT dummy AS val FROM system.one ) AS s1 ANY LEFT JOIN ( SELECT toLowCardinality(toNullable(dummy)) AS rval FROM system.one ) AS s2 ON (val + 100) = (rval * 7)"

Perf shouldn't be degraded, for example I did a simple non scientific series of time on 10k runs fuzzings and observed that.

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[robot-ch-test-poll4]

This is an automated comment for commit 453eb25 with description of existing statuses. It's updated for the latest CI running

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Check name	Description	Status
AST fuzzer	Runs randomly generated queries to catch program errors. The build type is optionally given in parenthesis. If it fails, ask a maintainer for help	❌ failure
CI running	A meta-check that indicates the running CI. Normally, it's in success or pending state. The failed status indicates some problems with the PR	⏳ pending
Stateless tests	Runs stateless functional tests for ClickHouse binaries built in various configurations -- release, debug, with sanitizers, etc	❌ failure

[tomershafir]

@alexey-milovidov can you help with CI failures here?

[alexey-milovidov]

Stateless tests (asan) [1/4] — fail: 1, passed: 1554, skipped: 15

This is a benign data race in curl: curl/curl#13065

AST fuzzer (debug) — Logical error: 'Expected single dictionary argument for function.'.

This is related to a non-released feature, "Analyzer".