core/rawdb: inspect database in parallel (#32506)

`db inspect` on the full database currently takes **30min+**, because
the db iterate was run in one thread, propose to split the key-space to
256 sub range, and assign them to the worker pool to speed up.

After the change, the time of running `db inspect --workers 16` reduced
to **10min**(the keyspace is not evenly distributed).

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Signed-off-by: jsvisa <[email protected]>
Co-authored-by: Gary Rong <[email protected]>

Author: jsvisa