`reconnect()` can produce partial resubscriptions

[BMDan]

In reconnect(), subscriptions are re-established:

Adafruit_CircuitPython_MiniMQTT/adafruit_minimqtt/adafruit_minimqtt.py

Lines 970 to 975 in bf47a0e

	if resub_topics and subscribed_topics:
	self.logger.debug("Attempting to resubscribe to previously subscribed topics.")
	self._subscribed_topics = []
	while subscribed_topics:
	feed = subscribed_topics.pop()
	self.subscribe(feed)

But there's a timing issue here; if the above code fails partway through re-subscribing, there's a silent failure case; witness:

# <connect, set on_message, etc.>
mqtt_client.subscribe("foo")
mqtt_client.subscribe("bar")

while True:
    try:
        mqtt_client.loop()
    except MMQTTException:
        while True:
            mqtt_client.reconnect()
            break
        except MMQTTException:
            # reconnect failed; wait a bit and try again
            time.sleep(5)

In this scenario, if a reconnect happens and subscribe("foo") succeeds but "bar" throws an exception, then _subscribed_topics will merrily report ["foo"], and the next reconnection attempt will subscribe to "foo" and consider its job complete, returning control to the main loop without ever reattempting "bar".

(Moved from its original home in #252, with h/t to this comment.)