Restrict libp2p_ping::handler::Failure::Other with Sync?

[drHuangMHT]

Other variant holds value of this type:

rust-libp2p/protocols/ping/src/handler.rs

Lines 97 to 99 in 4260702

	Other {
	error: Box<dyn std::error::Error + Send + 'static>,
	},

Which is not Sync, thus cannot be shared between threads, preventing it from being broadcast to subscribers.
Should we add Sync bound to it?

[thomaseizinger]

Yes that makes sense. I am wondering if this is a breaking change? I don't think it is so because we are adding a trait bound, not removing one.