Add events queue abstract class and implem

[mauropasse]

Here I create a pure virtual EventsQueue class and an implementation SimpleEventsQueue

Usage:

#include "rclcpp/experimental/buffers/simple_events_queue.hpp"

auto events_queue = std::make_unique<rclcpp::experimental::buffers::SimpleEventsQueue>();

auto executor = std::make_shared<rclcpp::executors::EventsExecutor>(std::move(events_queue));

Todo:
Implement in a different PR other policies

[mauropasse]

With my latest changes this PR could be merged.
This other PR to my own branch is useful to discuss a BoundedEventsQueue implementation:
mauropasse#6

[alsora]

why not pass this to constructor?

[mauropasse]

Did it on 5780ccf

[alsora]

The unit-tests for the queues should be independent from the executor.
You should just create a queue and start manually pushing/popping things

[mauropasse]

Ok I pushed this commit mauropasse@6fb2463
but this PR is not being updated, strange. I'll wait some time to see if gets updated