WIP: Fix notification unsubscribe lock

[Electro707]

Here is a description of the cause of this pull request (I will be referring to the bgapi backend).

Description of issue:

When the unsubscribe function tries to unsubscribe from a certain notification, it locks a thread lock with with self._lock. The unsubscribe function then calls a function which waits for a 'write confirmation' data packet from the Bluetooth client to confirm that the write to the notification configuration handler is successful. If a server is notifying the client at a fast enough pace, a notification packet will come it before the 'write confirmation' packet. That notification packet will then call the _receive_notification function. That function can't run as the _lock lock is locked up by the subscribe function. This results in a lockup, as the _receive_notification function is trying to use a lock that is occupied and won't be released until the subscribe function receives a 'write confirmation' packet, which it can't as the packet handler is locked up by the _receive_notification function.

(Hopefully I got my point across and didn't create more confusion than before)

Fix:

A solution is to have the _receive_notification function check if either:

• The _lock lock is acquired by another function (What this PR currently does as of commit 60b7ed4)
• Check if there is the handle it wants in the self._callbacks array before the with self._lock line is called.

What is the best way to approach this issue (or if there is another better way to)?