gc multiple indexes at once

[trinity-1686a]

Description

group deletion of splits by the gc. Before this, we'd do 2 list_splits per index every gc period (10m), now we do 2 list_splits in total (assuming pagination doesn't make us do more calls, in both cases). We can't group split deletion between index, so we still do n delete_splits, but n is the number of index with splits to delete, not the number of splits, so it ought to not be as bad.

2nd part of fix for #5353

How was this PR tested?

updated tests

[github-actions]

On SSD:

Average search latency is 1.01x that of the reference (lower is better).
Ref run id: 3248, ref commit: 80364dd
Link

On GCS:

Average search latency is 1.05x that of the reference (lower is better).
Ref run id: 3249, ref commit: 80364dd
Link