wt_blkcache_write contains a heuristic for whether it's worth compressing a block. Originally, this was whether the block was smaller than the minimum block allocation size. This makes sense, because we'd have to allocate the minimum block allocation size anyway.

But the presence of the disagg block manager changes that – this block manager doesn't allocate space in files so has no concept of a minimum allocation size. I changed the heuristic to be "double the header size" but that'll only work for disagg, and doesn't make sense for the existing block manager.

We should come up with a better heuristic, or change the heuristic based on the block manager, or something else (block manager to call to check if a page is worth compressing?)