Many python tests were setting the GC/compaction period to large
values, to effectively disable GC / compaction. Reserve value 0 to
mean "explicitly disabled". We also set them to 0 in unit tests now,
although currently, unit tests don't launch the background jobs at
all, so it won't have any effect.
Fixes https://github.com/neondatabase/neon/issues/2917
Replace the layer array and linear search with R-tree
So far, the in-memory layer map that holds information about layer
files that exist, has used a simple Vec, in no particular order, to
hold information about all the layers. That obviously doesn't scale
very well; with thousands of layer files the linear search was
consuming a lot of CPU. Replace it with a two-dimensional R-tree, with
Key and LSN ranges as the dimensions.
For the R-tree, use the 'rstar' crate. To be able to use that, we
convert the Keys and LSNs into 256-bit integers. 64 bits would be
enough to represent LSNs, and 128 bits would be enough to represent
Keys. However, we use 256 bits, because rstar internally performs
multiplication to calculate the area of rectangles, and the result of
multiplying two 128 bit integers doesn't necessarily fit in 128 bits,
causing integer overflow and, if overflow-checks are enabled,
panic. To avoid that, we use 256 bit integers.
Add a performance test that creates a lot of layer files, to
demonstrate the benefit.
