perf: flushing SQL stats to disk consumes significant cpu (1.5% cpu)

[ajstorm]

When running sysbench oltp_read_write we're seeing that disabling sql.stats.flush.enabled yields 1.5% higher QPS. We should investigate whether we can reduce this overhead and/or turn this off by default.

Jira issue: CRDB-44974

[blathers-crl]

Hi @ajstorm, please add branch-* labels to identify which branch(es) this C-bug affects.

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