Add Notary Actor to the morph Client's context

Is your feature request related to a problem? Please describe.

Notary actor is used by both storage node and IR node to send notary requests (ref. #2310). However, new actor is created each time the request need to be sent, which is awful.

Describe the solution you'd like

We need to create a single Notary actor once per morph client initialisation like it is done for RPC actor. However, Notary actor always contains committee multisignature account (either dummy or real, depends on IR/SN choice). Thus, we should track committee update events and create new Notary actor each time committee is changing.

Describe alternatives you've considered

https://github.com/neo-project/neo/issues/2763 would be very helpful, if implemented. Another option is to add some callback to Neo contract as we now have CN inside the IR node. And finally, we can always track committee members by polling.

Don't forget to add labels!

• component label: morph Client
• issue type: enhancement

Start by reviewing the morph Client and the RPC actor pattern, then follow the notary request flow referenced in #2310. The change is complete when one Notary actor is created per client initialization and a replacement is created whenever the committee changes.