[BUG] currentSpillableSize could cause OOM

[abellina]

The current check for currentSpillableSize where we want to make sure the store's spillable count is above the target size, can prevent us from spilling anything and makes big assumptions that what is in the store is not contributing to fragmentation.

We do not want to check currentSpillableSize at all, and instead we want to just spill at least one buffer and let the allocation retry (per @jlowe's comment here: #7572 (comment)).

Note that we also want to look at prioritizing the spillables based on size: #7705, which is related to the heuristic here.