satellite/accounting: Performance improvements to getNodeIds used by GetBandwidthSince

[ethanadams]

Change-Id: Ie94827697181c3bfe51c869460e5d0e2a1f50902

What:
Get distinct node IDs from storagenode_bandwidth_rollups since the last rollup timestamp, instead of from all rollups

Why:
The current query can take over 2 minutes on CRDB. This change makes it less than 1 second.

Please describe the tests:

• Test 1: Existing tests.
• Test 2:

Please describe the performance impact:

Code Review Checklist (to be filled out by reviewer)

• NEW: Are there any Satellite database migrations? Are they forwards and backwards compatible?
• Does the PR describe what changes are being made?
• Does the PR describe why the changes are being made?
• Does the code follow our style guide?
• Does the code follow our testing guide?
• Is the PR appropriately sized? (If it could be broken into smaller PRs it should be)
• Does the new code have enough tests? (every PR should have tests or justification otherwise. Bug-fix PRs especially)
• Does the new code have enough documentation that answers "how do I use it?" and "what does it do?"? (both source documentation and higher level, diagrams?)
• Does any documentation need updating?
• Do the database access patterns make sense?