fetcher: sanity check metas for common corruptions

[MichaHoffmann]

• [x] I added CHANGELOG entry for this change.
• [ ] Change is not relevant to the end user.

Changes

Mark metas with incomplete files as corrupted. Once shipper uploads the meta, the block is considered complete. This should make it possible to use the "Files" field in meta for a cheap check if the block is valid if its already in object storage. Neither storage-gw nor compactor can handle incomplete blocks, so this should fix a couple runtime issues; produce metrics and automatically clean up this issue.

Added a flag to disable cleanup of partially uploaded blocks that defaults to true.

Verification

Added unittests

Should solve #6328