fix:Race condition on instance variable assignment

[marcotc]

This PR fixes most recent CI timeouts, specially the ones seen for JRuby:

Failures:

  1) Datadog::Workers::Polling when included into a worker #perform by default 
     Failure/Error: raise StandardError, 'Wait time exhausted!' if attempts <= 0
     
     StandardError:
       Wait time exhausted!
     # ./spec/support/synchronization_helpers.rb:41:in `block in try_wait_until'
     # ./spec/support/synchronization_helpers.rb:36:in `try_wait_until'
     # ./spec/ddtrace/workers/polling_spec.rb:27:in `block in <main>'

These timeouts happen because probing worker.run_loop? can have the non-thread-safe side-effect of setting @run_loop = false:

def run_async?
  @run_async = false unless instance_variable_defined?(:@run_async)
  @run_async == true
end

If the check for instance_variable_defined?(:@run_async) is executed then the Ruby VM context switches to:

def perform_loop
  @run_loop = true

  loop do
    # ...
    return unless run_loop? || work_pending?
  end
end

which sets it to true: @run_loop = true.

Then context switches back and executes the assignment part of:

@run_async = false unless instance_variable_defined?(:@run_async)

@run_async will be set to false prematurely, effectively terminating the worker.

One alternative solution to the changes in this PR is to wrap #run_async? in a mutex, but this is not necessary, as the changes introduced here respect strict Happened-before semantics: we wait until instance_variable_defined?(:@run_async) returns true for us to ever evaluate @run_async, and assigning a value to @run_async is guaranteed to happen before instance_variable_defined?(:@run_async) returns true, so we now only read it when it's guaranteed to be available.
Also we stop assigning a value @run_async, which means this method does not have to worry about read/write race conditions that it could cause.

I've applied this fix to all instances of such pattern I could find in our code base.

[marcotc]

Pinging @DataDog/apm-ruby, as builds continue to fail (slowing productivity) due to reasons solved by this PR.