fix: empty set_waku_event_callback at shutdown, refactor mpsc sender

[hopeyen]

Description

• Removed force exit after waiting for shutdown
• Found the reason for hanging threads for MPSC receiver; set waku event callback to empty upon shutdown to release MPSC senders
• Unwrapped sender from Arc tokio::Mutex as the wrapper is unnecessary
• Shutdown should be within 3 seconds (might not work if discv5 is enabled, as waku::peermanager continues to handle peer connections, the issue has been raised with Waku team

Issue link (if applicable)

Resolves https://github.com/graphops/engineering-meta/issues/43

Checklist

• [x] Are tests up-to-date with the new changes?
• [x] Are docs up-to-date with the new changes? (Open PR on docs repo if necessary)

[pete-eiger]

just a small comment (it's noted in the description but still), with discv5 it sometimes works, but most times it hangs on Stop Waku gossip node