Connpool host-to-pool map is poorly structured

client/connpool.

is one of those places where we have poor C++ style and usage.
There's a std::map that uses a very unusual comparator that uses only part of the key. It's really abusing the std::map API and it's a lot of extra fancy footwork we don't need to do.

The implementation could be much more elegantly expressed in other ways as well, like using modern C++ like CTAD, auto, ranged-for, structured bindings, etc. It would be much easier to follow at that point.