Heterogeneous lookup in concurrent_hash_map

Extended concurrent_hash_map to support heterogeneous find/count/erase
operations. Our implementation is based on the idea described in ISO C++
paper P0919R3 for std::unordered_map
(http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0919r3.html).