fix: increase RocksDB background threads

[halfprice]

Description

Cherry-picks #3155 into the v1.44 release.

Test plan

See #3155.

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Release notes

• Storage node: Increases the number of RocksDB background threads from 2 (RocksDB default) to 16. Can be configured through the config file.

[github-actions]

Warning: This PR modifies one of the example config files. Please consider the
following:

• Make sure the changes are backwards compatible with the current configuration.
• Make sure any added parameters follow the conventions of the existing parameters; in
  particular, durations should take seconds or milliseconds using the naming convention
  _secs or _millis, respectively.
• If there are added optional parameter sections, it should be possible to specify them
  partially. A useful pattern there is to implement Default for the struct and derive
  #[serde(default)] on it, see BlobRecoveryConfig as an example.
• You may need to update the documentation to reflect the changes.