Fix --clear-db on Windows

[dv8silencer]

What type of PR is this?

Bug fix

What does this PR do? Why is it needed?
Fixes the issue where --clear-db in Windows leads to an error quoted in #7401 :

WARN node: Removing database
ERROR main: could not clear database: could not remove database file: remove C:\Users\Username\AppData\Roaming\Eth2\beaconchaindata/beaconchain.db: The process cannot access the file because it is being used by another process.
ERROR FAILED

This is in the validator, beacon-chain, as well as the slasher.

This is because on Linux, you can indeed os.Remove a file in use without the OS complaining-- this unlinks it from the containing directory but since a process has it open, the actual data on disk will remain until the process ends. At the time the process exits, if there is no other reference to that data, it'll be moved to the chopping block. On Windows, you cannot delete a file in use. In master, the clearing DB code calls a function to delete the .db file and then soon afterwards, creates a new DB [file] which will indeed persist after the process ends. But before it does the delete, it has a database with that file to-be-deleted DB file still open.

Example in the beacon-chain:

	if clearDBConfirmed || forceClearDB {
		log.Warning("Removing database")
		if err := d.ClearDB(); err != nil {
			return errors.Wrap(err, "could not clear database")
		}
		d, err = db.NewDB(dbPath, b.stateSummaryCache)
		if err != nil {
			return errors.Wrap(err, "could not create new database")
		}
	}

Which issues(s) does this PR fix?

Fixes #7401

Other notes for review

[codecov]

Codecov Report

Merging #7474 into master will not change coverage.
The diff coverage is n/a.

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  Coverage   60.60%   60.60%           
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  Files         424      424           
  Lines       30459    30459           
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  Hits        18461    18461           
  Misses       8989     8989           
  Partials     3009     3009           

[farazdagi]

Thank you, looks great!