perf: index asset_urn, timestamp cols in asset_probes tbl

[sudo-suhas]

By default, golang-migrate wraps multiple SQL statements in a transaction. Dropping index concurrently is not allowed in a transaction. So the drop statement needs to be the only statement in the migration.

These indexes are necessary because we order by and filter the rows in asset_probes with these columns:

• https://github.com/goto/compass/blob/v0.6.0/internal/store/postgres/asset_repository.go#L383-L389
• https://github.com/goto/compass/blob/v0.6.0/internal/store/postgres/asset_repository.go#L408-L428

[coveralls]

Pull Request Test Coverage Report for Build 5922869460

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 87.227%

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Totals
Change from base Build 5875638947:	0.0%
Covered Lines:	6194
Relevant Lines:	7101

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💛 - Coveralls