chain head listener: Fix race condition

[leoyvens]

This is a correction to #2365. There is a race condition in the else branch where the watcher could be overwritten, and a subscriber might end up with a receiver that has no sender and would error.

This keeps it simple by using a Mutex, and switching to a sync Mutex since that's what tokio recommends for short critical sections.

[evaporei]

I read all of the code and it seems fine 🙂
I just have one question, why wasn't RwLock working before? 🤔

[leoyvens]

@otaviopace We need to have a complete lock even for the read because otherwise two threads can read, not find an entry, and simultaneously insert a new watcher, and one will overwrite the other.