Change query cancellation mechanism

[skreborn]

By default, queries have a timeout, which means every query has to complete in a fairly arbitrary amount of time.

While settings the default timeout to a larger value or disabling it entirely is possible (and quite easy), I propose that a default of no timeout in addition to a manual cancellation method would be ultimately better.

Zenoh could, in this case, provide timeout as merely a convenience wrapper around executing the cancellation process itself after a given period has elapsed without a reply, while remaining unopinionated about the default mechanism.

Cancellation tokens (like the one in tokio-util) could be a nice way to manage this.

I understand that this would be a breaking change, but luckily only on the API side, not the protocol.