Simplify wakunode2 subscriptions model

[jm-clius]

Problem

In order to notify mounted protocols of new messages, wakunode2 makes use of subscriptions - an extendable map of protocol strings to handlers. This is used, for example, to notify the store of new messages or to check if a message match any registered filters.

This pattern works, but is unnecessarily confusing for two reasons:

1. It is difficult to read and follow due to the indirection it introduces. It should be possible to simply call notify on mounted protocols without having them register a handler in a separate map.
2. It overloads the idea of a subscription, which is also used to refer to topic subscriptions in the PubSub model.

Acceptance criteria

• something that simplifies the above, is clear to read and doesn't overload terminology