cpp: improve performace of MessagerMap whose key is string type

[wenchy]

loader/_lab/cpp/src/demo/hub.pc.h

Line 77 in e02236d

using MessagerMap = std::unordered_map<std::string, std::shared_ptr<Messager>>;

1. Maybe CRC32 is a good option: convert string to integer to improve hasher performance.
2. use std::array instead of std::unordered_map to boost lookup speed. (NOTE: how to generate array index -> messager name mapping?)

NOTE: more details need to be added...

[wenchy]

This issue has been resolved by #95