pageserver: size trigger for image layer generation

[skyzh]

Currently the criteria for whether to generate an image layer or not is based on the number of delta layers. We should consider to make it based on size.

1. Larger tenants don't seem to rely as much on image layers as smaller tenants. My assumption is that for larger tenants, they have more full page writes to the pageserver, and therefore we don't need to generate a lot of image layers (that covers the full key range) for them.
2. L0->L0 compaction alternative. feat(pageserver): l0->l0 compaction #10814 Because we generate image layers based on number of deltas, if we have a tenant that writes a small amount of data during every restart, we will quickly accumulate small L1 delta layers and trigger image generation.

If we change the trigger for image layer generation to size-based, we can solve both the problems. For (1), as L0 compaction gets triggered based on L0 size, we would get a fixed-size L1 levels every time. As larger tenants cover larger keyspaces, each image layer generation partition would contain fewer data, and we will wait for more delta layers before generating images. For (2), as we look at the L1 sizes, we won't generate images if L1 levels are small.