Move, not copy, Logger and Handler functors

[db-src]

Hi, and thanks a lot for the nice little library!

Here are a few minor improvements:

• Explicitly #include for use of std::move [rather than relying on indirect include]
• Move not copy Logger arg from Client to ClientImpl [will avoid copying heavy functors]
• Move not copy, set_error_handler Handler to lambda [ditto]
• Remove null statement in non-empty if/else block [I guess it was a relic from a time before the other statement was added.]

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We might also consider whether to move string() arguments such as these from arguments into their destination - what do you think?

[yhirose]

@db-src looks good to me. I'll merge it. As for string(), I am ok with the existing code. Thanks for the improvement!