The dns_resolver creates a lot of smaller objects (fetch context, fetch
counter, query, response, ...) and those are all loop-bound.
Previously, those objects were allocated from the a single resolver
context, which in turn increases contention between threads - remember
"dead by thousand atomic paper cuts". Instead of using a single memory
context, use the per-loop memory contexts that are bound to a specific
loop and thus there's no contention between them when doing the memory
accounting.
519481dcdb