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.