Replace enum FetchCMD by struct FetchCMD

[eval-exec]

What problem does this PR solve?

We don't realy need enum FetchCMD to record block fetch task, since it have only one enum item, the suggestion is to use a struct FetchCMD with peers and ibd_state fields. This modification can result in cleaner code.

What is changed and how it works?

What's Changed:

Related changes

• Replace enum FetchCMD by a struct

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code ci-runs-only: [ quick_checks,linters ]

Side effects

• None

Release note

None: Exclude this PR from the release note.