ConnPoolControl should provide list of known routes

Currently ConnPoolControl.getStats(HttpRoute) requires caller to store known routes somewhere to get complete detailed connections stats, for example, when checking for slow backends.

My suggestion is to add Set<T> getRoutes(); function to ConnPoolControl which will return snapshot of all routes served by this pool.
With this function it will be able to get all routes stats, detect the weak links among backends and report them.

I understand, that this change will break compatibility and won't be shipped with any 4.3.x version, but can we have this in 4.4?