daemon/worker: drop caching of kr_request mempools

This caused a huge increase in real memory usage in case of queries arriving to kresd while being disconnected from internet. The usage was slowly creeping up, even over 2G.

Interesting past commits: b350d38d and two preceding.

There apparently was no real memory leak. I assume that reusal of long-living mempools is risky in terms of memory fragmentation, though the extent of the issue surprised me very much. The issue seemed the same with normal glibc and jemalloc.

I generally dislike ad-hoc optimization attempts like these freelists. Now the allocator can better decide itself how to reuse memory.