Replace locked mempools with memory contexts

Replace locked mempools with memory contexts

Current mempools are kind of hybrid structures - they serve two
purposes:

 1. mempool with a lock is basically static sized allocator with
    pre-allocated free items

 2. mempool without a lock is a doubly-linked list of preallocated items

The first kind of usage could be easily replaced with jemalloc small
sized arena objects and thread-local caches.

The second usage not-so-much and we need to keep this (in
libdns:message.c) for performance reasons.