task-6329: refactor relay selection
task-6329: refactor relay selection

Instead of doing all our tests in RelayStats.get_relay(), we now have a set of
filters, each of them with its own class, that will be added to a filter chain
depending on command-line options.

gsathya/metrics-tasks,sysrqb/metrics-tasks,gsathya/metrics-tasks,gsathya/metrics-tasks,gsathya/metrics-tasks,sysrqb/metrics-tasks,gsathya/metrics-tasks,sysrqb/metrics-tasks,sysrqb/metrics-tasks,sysrqb/metrics-tasks,sysrqb/metrics-tasks,gsathya/metrics-tasks