Enable limited use of asyncio with executors

[sloretz]

Resolves #962
Alternative to #963

This enables limited use of asyncio primitives with the rclpy executor. The most important parts are the changes to how Future instances handle __await__ and how Task instances get scheduled. They allow Task to tell when an asyncio future is awaited, and they allow Task to pause itself until the asyncio future completes. A consequence of this is Task now requires an executor.

There are limitations coming from asyncio not being thread safe.

• There must be an asyncio event loop running
• A single threaded executor must be used, and that executor must spin periodically in the same thread as asyncio's event loop

Changes

• rclpy Future.__await__ yields itself instead of None - this allows Task to know if an rclpy Future is being awaited
• Add Executor.call_soon() - This is like create_task(), but if the callback is already a task then it won't create one. The naming is taking from asyncio
• Executor._tasks is cleared every time tasks are run. Task instances schedule themselves when they become unblocked.
• If Task detects it's waiting on an asyncio future, then it will make the asyncio future schedule the Task when it's done
• Some tests had asyncio.sleep(0) in them already, but didn't create an asycnio event loop. Those uses have been removed.
• I removed Task._lock because it should never run in parallel now that it schedules itself.
• I removed Task.executing() because it would have needed the Task._lock, and wasn't used anywhere else.

[sloretz]

This PR also seems to be a small performance improvement. Using the benchmarks from #973 (1527a9e) and running each test 5 times on each branch there's improvement in every single test. The average improvement is 1.75%.

Benchmark	Timer	Timer coro	Guard	Guard coro	pub/sub	client/server	action
master	162.1116	162.1116	159.8493	158.453	242.1545	528.9525	680.5634
	166.2661	166.513	166.1291	167.0961	237.8923	524.6592	658.3939
	164.1507	164.2656	164.2676	164.256	234.6498	527.6705	679.2866
	165.3802	164.6015	162.6969	161.9369	237.4209	516.9322	683.0678
	165.8009	165.1686	164.9115	165.091	237.8524	511.6613	691.9759
Average	164.7419	164.53206	163.57088	163.3666	237.99398	521.97514	678.65752
with 971	159.8871	159.4097	160.1809	163.3935	228.803	517.9435	655.3708
	163.6177	163.1115	165.7945	165.0023	224.5216	523.299	691.9127
	162.6403	158.5519	159.661	158.085	235.0898	518.9754	666.6267
	160.8633	161.9503	158.0991	163.3056	234.2222	520.8381	667.9764
	163.3952	163.5464	156.8233	156.1533	224.0261	521.4153	666.8869
Average	162.08072	161.31396	160.11176	161.18794	229.33254	520.49426	669.7547
Percent diff	-1.615363183	-1.955910599	-2.114752944	-1.33360185	-3.639352558	-0.283706998	-1.311828093
Average	-1.750645175

[sloretz]

CI (build: --packages-above-and-dependencies rclpy test: --packages-above rclpy)

• Linux
• Linux-aarch64
• Windows

[Achllle]

@sloretz Do you still see this approach as the best way forward? It would be helpful to know what the best approach is to marry asyncio and rclpy as there doesn't seem to be one currently, and this PR seems to be in limbo.