Rework how services are started/stopped

[davidbrochart]

Description

pycrdt-websocket provides these 4 services:

• WebsocketServer: a WebSocket server.
• WebsocketProvider: a WebSocker provider (a client to a WebsocketServer).
• YStore: a store for persisting Y updates.
• YRoom: a room where clients collaborate on a Y document.

These services are asynchronous, and have to be started (for initialization) and stopped (for teardown). For instance, initialization could include connecting to a database, and teardown could disconnect from the database.
They provide 2 common APIs to be started and stopped:

• an async context manager.
• individual start() and stop() async methods.

The async context manager API looks like this:

async def main():
    async with WebsocketServer() as websocket_server:
        ...

And the start/stop API looks like this (using AnyIO):

async def main():
    websocket_server = WebsocketServer()
    async with create_task_group() as tg:
        await tg.start(websocket_server.start)
        ...
        await websocket_server.stop()

In an environment where it is not possible to use AnyIO task groups, once can use asyncio like this:

async def main():
    websocket_server = WebsocketServer()
    task = asyncio.create_task(websocket_server.start())
    await websocket_server.started.wait()
    ...
    await websocket_server.stop()

While it is advised to use the async context manager API, it is not always possible to do so, hence the manual start/stop methods.

Improvements

The way services are started and stopped now uses an anyio.Lock, instead of a boolean attribute _starting, to handle concurrent calls. Also, __aenter__() calls start() under the hood, and __aexit__() calls stop(), instead of copying code. This ensures that both ways are equivalent. The start() method is passed a parameter from_context_manager=True if called from __aenter__(), in order to not create a new task group, since it was already created in __aenter__().

Tests have also been reworked, so that both APIs (context manager and start/stop methods) of every service are tested. Also, since #22 both asyncio and trio backends are tested.

Breaking changes

This PR has the following breaking changes:

• the stop() methods are now async. It didn't make a lot of sense to have start() async and not stop(), and teardown could have async logic in general.
• WebsocketServer.delete_room() is now async, since it may stop a room, and stopping a room is async.