[Indexer] A few improvements to backfill tool

[lxfind]

Description

This PR adds 3 improvements to the sql backfill tool:

1. It allows ON CONFLICT DO NOTHING, so that we can safely backfill gaps without been too precise.
2. It tunes down the default concurrency and chunk size, and allows for override through command line args.
3. It prints out the minimum in-progress checkpoint so that if it ever stops for some reason, you can restart using that number.

Test plan

Run again locally

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Release notes

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• Protocol:
• Nodes (Validators and Full nodes):
• Indexer:
• JSON-RPC:
• GraphQL:
• CLI:
• Rust SDK:
• REST API:

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[amnn]

Thanks @lxfind !