bluez: Create a bluez manager instance per-event-loop

[projectgus]

Alternative to #1031. Avoids "got Future attached to a different loop" if Bleak instances with BlueZ backend are constructed in context of more than one event loop (for example, in default configuration of pytest-asyncio).

Also enables parallel access to Bleak instances from multiple event loops, provided the same Bleak object is not shared between two event loops. Each backend manager instance establishes an independent dbus connection with a unique dbus address.

Some quick code that demonstrates using Bleak in parallel from multiple threads (and therefore multiple event loops):

import asyncio
import logging
from logging import getLogger, getLevelName, Formatter, StreamHandler
import threading
import time

from bleak import BleakScanner

log = getLogger()
log.setLevel(getLevelName('INFO'))
log_formatter = Formatter("%(asctime)s [%(levelname)s] %(name)s: %(message)s [%(threadName)s] ")
console_handler = StreamHandler()
console_handler.setFormatter(log_formatter)
log.addHandler(console_handler)

def scanner_callback(device, adv_data):
    log.info(f"Callback Loop {id(asyncio.get_running_loop())} device {device}")

async def scan_coro():
    log.info(f"Scanning Loop {id(asyncio.get_running_loop())}")
    async with BleakScanner(scanner_callback):
        await asyncio.sleep(2)
    log.info("Coro done")

def run_scanner():
    log.info("Running thread")
    asyncio.run(scan_coro(), debug=True)
    log.info("Thread done")

threads = [ threading.Thread(target=run_scanner),
            threading.Thread(target=run_scanner),
            threading.Thread(target=run_scanner), ]

for t in threads:
    t.start()
    # note need to stagger starting scanning as BlueZ seems to drop responses to StartDiscovery if they happen at
    # the same time (seems to be a BlueZ limitation or bug, not anything Bleak is doing).
    time.sleep(0.4) 

[t.join() for t in threads]

There is also a simple pytest-asyncio case in PR #1031 that fails without this patch and passes with this patch.

[dlech]

Looks good. Just a few suggestions.

[projectgus]

@dlech Thanks for the quick and thorough review! Updated accordingly.

[dlech]

Thanks!