Fixes timing leaks in benchmarks with large setup operations (e.g.,
clearing 27k staking entries). After bulk deletions are committed, the
first new allocation can trigger memory allocator overhead that leaks
into benchmark timing.

The fix adds a memory allocator warmup step in `commit_db()` that
performs a dummy write/clear cycle to absorb this overhead before timing
starts.

Fix #10798.

Another related issue:
#10813 (rework of
staking benchmarks to avoid massive bulk deletion if not needed. An
example showing the validity of the approach
[here](#10822 (comment))
where we just remove a clear_validators_and_nominators() from one
benchmark and that's enough to go down from ms to microsec)

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Co-authored-by: cmd[bot] <41898282+github-actions[bot]@users.noreply.github.com>