Cleanup socket allocation logic in cluster info (and in tests in general)

[alexpyattaev]

Problem

#5824

• Socket binding logic is a disaster of code duplication (mostly related to codestle and a hack to store RPC ports) with sudden flakiness mixed in (since multiple tests can be trying to book the same port ranges at the same time).
• Tests would flake because we were relying on port binding logic to never try to reuse the same port twice (we'd look up a free port and still expect it to be free 5 seconds later all the while running 64 tests in parallel)

Summary of Changes

• Merge the socket binding code into more coherent units, move some of the logic into net-utils for clarity (-200 LOC of boilerplate)
• Add function to net-utils that can allocate 20-port slices that are unique across all tests currently running
• Make all validator instances in unittests use non-intersecting port blocks using nextest environment variables to dedup them.