Add retrieve(), slightly faster than find when you need a value, not iterator.
Also align bins to cache lines for very slight extra multithread speed.
ustring -- use unordered_map_concurrent, much faster with many threads.
Slightly slower with just 1 thread, break even with 2 threads.
Much faster with >= 4 threads, have seen 25% or more speed gain in benchmarks
involving many threads creating ustrings with high frequency.
Also use cache alignment to eliminate false sharing for the statistics counts.