Improved polymorphism in rep_crawler::cleanup_reps()

[brandon-bb]

Addresses #3946

This solution is an implementation of the proposed one by @simpago - channel state is represented using a boolean variable. tcp_channels::purge() toggles this variable to close the channel. As this was crafted to introduce minimal changes, there could be a better way to represent channel state.

[gr0vity-dev]

I had a quick look at the test results. It doesn't seem to pass the unit tests yet. Also when spinning up a local nano network with this version, most of the blocks don't confirm.

Taken from https://ct.bnano.info :

[dsiganos]

Not relevant anymore after the rep_crawler overhaul and recent networking improvements.