[sgen] Adjust free-ing of empty blocks

[sgen] Adjust free-ing of empty blocks

After sweep, depending on the computed allowance, we are freeing empty blocks so that we still keep around enough blocks to fill that allowance (to avoid repeated vmaps).

On both collectors this had the problem that the allowance also accounts for LOS space, which we shouldn't account for in major section heuristics (which means that in LOS intensive benchmarks we only free major sections after the LOS shrinks).

On the concurrent collector, where we have a negative allowance adjustment depending on the duration of the concurrent mark (in order to mimic serial memory usage), we can have very small allowances at the end of a major collection, even 0. This means that we would reserve no empty blocks for later use.

We solve these issues by reserving empty blocks relative to the amount of current alive blocks, which is also the basis of how we compute the allowance in the first place.