mod_cache_socache: avoid pool to heap reallocation.
Below some threshold, the previous code tried free (sub-)pooled memory ASAP by moving small buffers (< capacity / 2) to a heap bucket. But this is not really an optimization because first it requires at some point to allocate more than the configured capacity, and second since this happens during response handling the pool is about to be destroyed soon anymay.
This commit simply keeps the data in the subpool and uses a pool bucket for the output brigade to take care of the lifetime until it's consumed (or not).
Follow up to r1853874: CHANGES entry. Submitted by: ylavic Reviewed by: ylavic, icing, jim