yaqd-core: container to hold asyncio task references

[ddkohler]

For asyncio, I believe best practice is to create a reference to all tasks you create to avoid garbage collection e.g. for a daemon instance:

task = self._loop.create_task(some_coro)

Often, especially for short-lived tasks, we don't reference. I am unsure how important this practice actually is, but this has bitten us in the past (see, e.g. #77 ).

A simple object to keep track of these things e.g.

import asyncio
from typing import Coroutine


class TaskSet(set):
    """container class for tasks to keep strong references"""
    def add(self, task:asyncio.Task):
        super().add(task)
        task.add_done_callback(self.discard)

    def from_coro(self, coro:Coroutine):
        """create task and add to running loop"""
        task = asyncio.get_running_loop().create_task(coro)
        self.add(task)

might be of general use? Would a helper class like this be useful to include in yaqd-core?

Such an item could be implemented for IsDaemon._tasks, but _tasks is currently a list, so (no change would give no support for append). Choice of set or list is probably not a big deal...