Make destroy_subscription thread safe

[sloretz]

Resolves #255

This makes node.destroy_subscription thread safe by wrapping the pycapsule with a class that keeps track of who is using it. The subscription is destroyed when it is no longer being used.

Potentially breaking changes

• _rclpy.rclpy_create_subscription() returns just the pycapsule instead of a tuple. However, I would be surprised if any downstream projects are using this method.
• _rclpy.rclpy_destroy_node_entity() no longer destroys subscriptions. Instead rclpy_pycapsule_destroy() does the work. Again I would be surprised if any downstream code is using this.
• The class Subscription no longer has attributes subscription_handle and subscription_pointer. Instead it just has handle. I would expect those attributes to have been for internal use only.

Next steps

In order to keep PRs small, I would like this PR to get review and be merged after addressing feedback. Afterwards I'll create a new PR to fix the same thread safety issues for the other destroy methods:

• destroy_publisher
• destroy_client
• destroy_service
• destroy_timer
• destroy_guard_condition

Finally I want to defer making destroy_node thread safe, and instead create an issue for it. It requires more thought since it can't be destroyed until every entity created from is no longer being used and has been destroyed.

[sloretz]

CI (testing packages above rclpy)

• Linux (test_params_yaml test_cli.test_params_yaml flaky build_farmer#166)
• Linux-aarch64 (test_params_yaml test_cli.test_params_yaml flaky build_farmer#166)
• macOS Some sort of CI machine failure, investigating

  --- stderr: intra_process_demo
  13:24:37 make: *** [all] Alarm clock: 14
  13:24:37 make: INTERNAL: Exiting with 1 jobserver tokens available; should be 4!
  13:24:37 ---
  13:24:37 Failed   <<< intra_process_demo	[ Exited with code 2 ]
  13:24:48 
  

  • re-run
• Windows failed to delete workspace Job failed to delete workspace build_farmer#161
  • Windows (test_params_yaml test_cli.test_params_yaml flaky build_farmer#166)

[wjwwood]

I had a few comments, but in general lgtm.

[sloretz]

CI (testing just rclpy, mostly to check that _Static_assert is supported on all platforms)

• Linux
• Linux-aarch64
• macOS
• Windows

[sloretz]

CI with 69399f5 testing packages above rclpy to make sure the new commits didn't break anything

• Linux
• Linux-aarch64 (test_params_yaml test_cli.test_params_yaml flaky build_farmer#166)
• macOS
• Windows (test_params_yaml test_cli.test_params_yaml flaky build_farmer#166)