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Before this commit, we had one small buffer telling the packet type and length, and another buffer with the payload. They were send to the kernel one by one. For small packets, this is a problem, as NODELAY causes the first buffer to be send on the IP stack, and the payload after. This increases the IP overhead for no good reason. Now instead, already reserve room in the packet to write the header, and send it as one single unit to the kernel.
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send() no longer is blocking, and all sendNNN calls now return false if the call couldn't be executed. Additionally, the library now recovers much better from issues, like unexpected broker disconnects.
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This avoids copying the pointers from Client::Impl into Connection, which is just administrative work. Now we can access Client::Impl, and have all variables available to us.
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Subscriptions are now stored in a tree-like structure, to quickly find the correct callbacks. This not only reduces the complexity from O(n) to O(logn), but also doesn't require stuff like regex. It does however require slightly more memory.
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std::move() is doing the right thing either way.
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This includes CONNACK and SUBACK.