fix: _bucket system table used to be created with different ids on different replicasets

Blocked by https://git.picodata.io/picodata/picodata/vshard/-/merge_requests/3

Summary

• fix: _bucket system table used to be created with different ids on different replicasets

Closes #1130 (closed)

• fix: don't ignore failed global DML

• refactor: extract some code from storage.rs

Add new files:

• storage/snapshot.rs: stuff related to raft snapshot
• storage/schema.rs: stuff related to DDL logic

Code is only moved, no changes to logic

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• Close #1130 (closed)
• Close #1128 (closed)
• Cherry-pick to: 24.6
• Docs follow-up: not necessary
• Changelog is updated