Peter M. Groen / TrueMQTT-cpp

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.

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.

std::move() is doing the right thing either way.

This includes CONNACK and SUBACK.