bits and bobs - fix me!
concurrent_hash_map first pass
Aggregation of list operations (in a manual way).
LRUCache using concurrent_unordered_map
Added unit test for LRUCache::get().
Removed unused LRUCache typedefs.
Tidied up LRUCache::setInternal().

Removed argument that is unnecessary now we use a concurrent_unordered_map internally.
Removed unused includes from LRUCache.
Tidied up LRUCache::eraseInternal().
Reordered LRUCache methods for improved clarity.

It's better if they're in the order people are likely to use them. Also added a bit more documentation.
Documented LRUCache private implementation.
Renamed LRUCache Ptr -> Value.

There is no requirement that it's a pointer type - that is a relic of an early implementation.
Added mutex to LRUCache::cached() for completeness.

Also added worrying note into limitCost().