Fixes for audio transaction history backfilling

[dharit-tan]

Description

Fixes 2 major bugs in audio_transactions_history table backfilling (which are unfortunately already deployed):

• Lock timeouts - originally there was a timeout on the indexing task lock which would allow a new task to grab the lock and begin indexing the same data. This would result in a race condition - one task would write out data to audio_transactions_history and whatever backfilling txs table it was using, then the other would try to write duplicate data and fail. Solution was to remove the timeouts.
• Solana flakiness - calls to get_signatures_for_address was returning an empty list even though there were definitely prior signatures for the given program, even after increasing retries 10x. The indexer logic would erroneously treat this as the "beginning of time" for that program, and stop indexing at that point resulting in sometimes huge gaps in the indexed data. The solution was to hardcode the min_sig value (lowest known signature for the program) and check against that - if we encountered an empty list when we shouldn't have, fail out and restart the task.
• Another small bug was a KeyError in helpers.py - switched to a for loop instead of using next.

Also added a db migration to delete all data associated with the backfilling so we can have a fresh start.

Tests

Tested on remote-dev with 3 DNs this time so we can check data consistency between them. Also tested on a prod sandbox.

Monitoring - How will this change be monitored? Are there sufficient logs / alerts?

Lots of logs, also switched to logger.info in index_spl_token_backfill.py for better visibility.

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[raymondjacobson]

@dharit-tan do we care about this PR still?

[dharit-tan]

@dharit-tan do we care about this PR still?

Nope, this code is deleted!