Throttle limiter assumes a single process environment

[adamruzicka]

Steps to reproduce:

1. Apply the attached patch[1]
2. Make all processes use the same DB by setting DB_CONN_STRING env variable
3. Run an orchestrator bundle exec sidekiq -c 1 -q dynflow_orchestrator -r ./examples/remote_executor.rb
4. Run a worker bundle exec sidekiq -c 5 -q default -r ./examples/remote_executor.rb
5. Run another worker bundle exec sidekiq -c 5 -q default -r ./examples/remote_executor.rb
6. Trigger the bug bundle exec ruby examples/remote_executor.rb client

The job will most likely fail with the following

undefined method `wait' for nil:NilClass (NoMethodError)
lib/dynflow/throttle_limiter.rb:71:in `block (2 levels) in handle_plans'
lib/dynflow/throttle_limiter.rb:69:in `tap'
lib/dynflow/throttle_limiter.rb:69:in `block in handle_plans'
lib/dynflow/throttle_limiter.rb:68:in `map'
lib/dynflow/throttle_limiter.rb:68:in `handle_plans'
[ concurrent-ruby ]
lib/dynflow/throttle_limiter.rb:21:in `handle_plans!'
lib/dynflow/action/with_sub_plans.rb:92:in `trigger_with_concurrency_control'
lib/dynflow/action/with_sub_plans.rb:82:in `trigger'

If not, stop all the processes, recreate the database and try again.

[1] - 0001-Reproducer-for-bug-with-concurrency-control-with-mul.patch