Replace concurrent-ruby with MonitorMixin

[bschaeffer]

concurrent-ruby isn't needed just to do thread safe hashes. Prompted by this memory profile where including the gem is adding almost 6MB at startup:

stoplight: 5.8203 MiB
    stoplight/data_store/memory: 5.293 MiB
      concurrent: 5.2383 MiB
        concurrent/executors: 1.3906 MiB
          concurrent/executor/timer_set: 0.6953 MiB
            concurrent/scheduled_task: 0.4844 MiB (Also required by: concurrent, concurrent/timer_task)
              concurrent/ivar: 0.3047 MiB (Also required by: concurrent/async, concurrent/future, and 2 others)
        concurrent/configuration: 1.3203 MiB (Also required by: concurrent/scheduled_task, concurrent/options, and 2 others)
          concurrent/delay: 0.8789 MiB (Also required by: concurrent/utility/processor_counter, concurrent)
            concurrent/concern/obligation: 0.7031 MiB (Also required by: concurrent/ivar)
              concurrent/atomic/event: 0.6094 MiB (Also required by: concurrent/executor/immediate_executor, concurrent/atomics, and 3 others)
                concurrent/synchronization: 0.5703 MiB (Also required by: concurrent/executor/abstract_executor_service, concurrent/utility/at_exit, and 29 others)
        concurrent/atomics: 0.7422 MiB (Also required by: concurrent/executor/simple_executor_service)

[bschaeffer]

Not sure why coverage dropped 4%... removed 15 lines should have gone up.

[tfausak]

Looks great, thanks!