feat: Notification monitoring system

[realvarx]

- What I did
Implement AtMonitorConnection and notification monitoring system.

- How I did it

• Took AtMonitorConnection.java as an example and implemented it, but used locks instead of synchronized (because synchronized doesn't exist in Python, it needs to be created as a decorator).

• Created AtEventType enum and implemented an event system based on Queue library. There were EventBus solutions based on asyncio lib, but I didn't find practical the added complexity of mixing Threading and asyncio for now.

• Created AtEvent class, which is essentially a wrapper for event + data associated to the event. AtEvent objects are then introduced into the Queue and read by other thread which will use that data.

• Added new parameters which will be eventually necessary to Metadata class (although they are commented because there's a circular import issue).

• TimeUtil to manage milliseconds, SocketUtil to create readline() method for SSLSocket.

• Created AtNotification class, although it isn't used for now.

• Added start_monitor and stop_monitor methods to AtClient class, which can be used to start the monitoring for notifications. Added handle_event method to AtClient class.

• atconstants.py is simply a file to define constants or global variables, such as locks or queues.

• Updated repl.py, which now includes a practical example of how this new feature can be used (simply create a new thread which calls a method, and handle the event reception through queues inside that method, also calling inside of it the handle_event method of AtClient class).

• Small tweaks not mentioned, feel free to ask.

• Issues:

1. Metadata circular imports exception if new code is uncommented.
2. Unexpected MONITOR_EXCEPTION events triggered when reading @atSign@notification notifications from yourself (doesn't happen with normal notification notifications also from yourself, which are actually the same as the other mentioned).
3. Current way to manage events with handle_event methods could be improved to a more abstract structure, although they work for now.

- How to verify it

1. Run repl.py and choose an atSign using option 1
2. Select option 2. REPL will start and activate monitor mode automatically in a different thread. You can still send commands/verbs. You will start seeing your own notifications (from yourself to yourself) and heartbeat working (noop verb is sent from time to time as a keepalive)
3. Use at_talk or any other tool to send notifications to your atSign from a different atSign. You should be able to see the complete notification, and the encrypted and decrypted value of it.

- Description for the changelog

feat: Notification monitoring system

Progress in #8 and #9

[cpswan]

Please remove all the commented code from metadata.py

The README should also be updated to provide an example of receiving a notification. It would be good to have some example code too, and some tests.

[cpswan]

Essentially the stuff above in the How to verify it section should be finding its way into more obvious docs (like the README) rather than being buried in this PR.

[Xlin123]

LGTM