This commit subclasses the paho MQTT client code (somewhat intrusively),
replacing its threading with anyio-style tasks.

Recursion is bad for you. More to the point it's not necessary.

Also, several common non-matches can be checked for up front.

Pre-split the topic so the comparison code doesn't need to run `split`
each time it's called.

Drop the trailing hash from the split wildcard topic.

This further simplifies the code and prepares for the next bit.

Subscriptions are arranged into a tree that's traversed
when a message is dispatched.

No sense in doing this in the comparator

Use `contextlib.aclosing` to cleanly stop the iterator

instead, synchronize via task_status.started()

If the server is busy it might delay forwarding messages
until after it sees the unsubscription.

anyio 4.4 warns when that doesn't happen

This change adds subscriptions with separate message queues.

Does not yet handle duplicate subscriptions.

needs just a bool return, not a counter

This test doesn't yet support subscribing to a topic multiple times.

TODO.

This change skips walking the topic tree, assuming server support.

More fixes required …

turns out that it obfuscates more than it helps, at least here