backport: Use range-based for loops (C++11) when looping over map ele…

[fdoving]

…ments

manual backport of bitcoin #10493 picked: 680bc2cbb34d6bedd0e64b17d0555216572be4c8

Origial-author: practicalswift

Before this commit:

for (std::map<T1, T2>::iterator x = y.begin(); x != y.end(); ++x) { }

After this commit:

for (auto& x : y) { }