pageserver: generate image layers at L0/in-mem boundary (or L0/L1 boundary)

[skyzh]

previous attempt failed #10548 because there are cases we didn't consider, which caused errors in staging:

• What if the LSN we selected doesn't contain any data (below all layers generated), we will get an error immediately saying DB_DIR key not exist.
• What if we select an LSN to generate image layers that is below an existing image layer, the we might get a read error in the middle if GC decides to remove layers below the existing image layer.

Currently, we generate image layers at last_record_lsn, which causes some problems,

• Future layers. So we would put an image layer that is above the remote/disk_consistent_lsn into the layer map, which adds a lot of complexity in the code path.
• Overlap between in-memory and image layers needs extra work to support, and pageserver: handle in-memory layer overlaps with persistent layers #11000 fixed the case.
• Make image layer generation more costly, if we have a lot of L0 layers between last_record_lsn and L0/L1 boundary. Every read will go through a lot of layers, versus we could have done a L0 compaction and then do image generation.

I'd like to continue to move image layer generation to a lower LSN and I think it's beneficial to do so.