public CompoundName append(CompoundName prefix, String suffix) {
        return cache.computeIfAbsent(prefix, __ -> new HashMap<>()).computeIfAbsent(suffix, prefix::append);
    }
Cache for compound names created through {@link CompoundName#append(String)}.
Creating new {@link CompoundName}s can be expensive, and since they are immutable, they
are safe to cache and reuse. Use this if you will create <em>a lot</em> of them, by appending suffixes.

@author jonmv
container-search/src/main/java/com/yahoo/search/query/profile/CompoundNameChildCache.java