Allow adding Pyrogram handlers during execution of a Pyrogram callback

[JosXa]

Pyrogram has a problem where it uses an asyncio mechanism to first acquire the locks responsible for safe handler manipulations when calling client.add_handler, but at least one of these locks will be used when Pyrogram executes a callback.
This means that there is categorically a deadlock when attempting to add a handler inside of a handler callback. Dan's way around that is to have an async helper function to acquire all locks and then add the handler, which is executed as fire-and-forget (asynchronously):

https://github.com/pyrogram/pyrogram/blob/332468d271896e2c81b1a283b6a0f14fc1a666c1/pyrogram/dispatcher.py#L137-L152

This however means that client.add_handler will always have effect only after the dispatcher is done executing the current callback, making it unusable for tgintegration. For the 1.0 release, this was not an issue as it's only advertised as a test framework and automation tool and not for userbots, but should be looked into in the future.