EC2 per-pool expiry path re-buys replacement RIs bought inside the coverage lookback #346

The EC2 per-pool expiry path (providers/aws/recommendations/expiry.go) deducts expiring RIs from lagged Cost Explorer coverage without crediting replacement RIs the account already bought for them. Cost Explorer coverage is a lookback average, so RIs started inside the lookback are not in it yet, and re-running the CLI within the lookback buys the replacements again.

The RDS family path had the same behaviour (found in the cli#2175 gate, F1) and is fixed in the linked PR with ApplyFamilyNUSizingRDSWithExpiryOptions: visible active, non-expiring RIs that started inside the coverage lookback are credited.

To do: check the EC2 per-pool deduction for the same re-run behaviour, with a regression test (expiring RIs, replacements bought a few days ago, coverage unchanged), and fix it with an explicit lookback parameter, additively.

Refs LeanerCloud/cloud-commitments-cli#2132, LeanerCloud/cloud-commitments-cli#2175.