[ENH] Bootstrap a wal3 log from existing content.

[rescrv]

Description of changes

This adds a "bootstrap" call to wal3 that copies the data semi-atomically. It is reasoned to be
safe to do concurrent with all other log operations, but may leave a FragmentSeqNo(1) lying around
in the event that a log is initialized at the same time it's bootstrapping. This should never
happen in our system so thankfully we can just leave the hole.

See #4558 for the reasoning.

Test plan

• Tests pass locally with pytest for python, yarn test for js, cargo test for rust

Documentation Changes

N/A

[github-actions]

Reviewer Checklist

Please leverage this checklist to ensure your code review is thorough before approving

Testing, Bugs, Errors, Logs, Documentation

• Can you think of any use case in which the code does not behave as intended? Have they been tested?
• Can you think of any inputs or external events that could break the code? Is user input validated and safe? Have they been tested?
• If appropriate, are there adequate property based tests?
• If appropriate, are there adequate unit tests?
• Should any logging, debugging, tracing information be added or removed?
• Are error messages user-friendly?
• Have all documentation changes needed been made?
• Have all non-obvious changes been commented?

System Compatibility

• Are there any potential impacts on other parts of the system or backward compatibility?
• Does this change intersect with any items on our roadmap, and if so, is there a plan for fitting them together?

Quality

• Is this code of a unexpectedly high quality (Readability, Modularity, Intuitiveness)

[propel-code-bot]

Bootstrap WAL3 Log from Existing Content

This PR adds a 'bootstrap' function to WAL3 that enables initializing a new log with a batch of existing data. The implementation copies data semi-atomically and is designed to be safe to use concurrently with other log operations. Though there's a potential edge case where a FragmentSeqNo(1) hole might be created during simultaneous initialization and bootstrapping, this is deemed acceptable as it shouldn't occur in the system's normal operation.

Key Changes:
• Added a new bootstrap method to copy existing content to a new WAL3 log
• Implemented semi-atomic data copy that's safe to run concurrently with other operations
• Added reasoning and safety comments throughout the code

Affected Areas:
• rust/wal3/src/writer.rs

This summary was automatically generated by @propel-code-bot