fix: update leveldb dependency

[mfmarche]

Update https://github.com/syndtr/goleveldb dependency

Fixes #349

Types of changes:

• Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Description of the changes being introduced by the pull request:

Updates the dependency to https://github.com/syndtr/goleveldb. leveldb required an fsync after creation of the MANIFEST file.
Please verify and check that the pull request fulfills the following requirements:

• Tests have been added for the bug fix or new feature
• Docs have been added for the bug fix or new feature

[joshuagl]

Thank you for submitting this fix @mfmarche! The CI appears to be failing due to a missing entry in go.sum https://github.com/theupdateframework/go-tuf/runs/7641572216?check_suite_focus=true#step:6:27 would it be possible to address that?

Also, the DCO check failed. Could you git commit --signoff (or git commit -s) to include a signed-off-by entry in your commits? (You'll want to fix the existing commit before you add new ones to address the failing CI, you can do that with git commit --amend -s on the branch you used).

[mfmarche]

hi @joshuagl thanks for the review. I made the changes and signed off. thank you.

[joshuagl]

LGTM, thanks again @mfmarche

[joshuagl]

Could someone give this a second +1? @znewman01 @asraa @mnm678