Consider std::move-ing callbacks into the command list instead of const references

[nickolasrossi]

Because the callbacks are passed in as const references, the final step where the callback is inserted into the command queue results in a copy. This is expensive if the callback is a functor object with a lot of state.

m_commands.push({redis_cmd, callback});

If the callbacks were instead passed by value, and forwarded into the command queue via std::move, C++17 copy elision would eliminate the copies.

m_commands.emplace(redis_cmd, std::move(callback));

This sort of thing is typical with asio callbacks and could be applied here as well.