sync: handle panic during mpsc drop

[mox692]

This PR updates the mpsc to handle panics during the drop. Specifically, the chan::Rx will continue executing its drop even if a panic occurs during the drop.

Although this scenario is expected to be rare, there could be edge cases, for example:

• On thread1, chan::Rx is dropped
• On thread1, panic occurs in the middle of the drop
• On thread2, chan::Chan is dropped (resuming from the interrupted element)
• On thread2, panic occurs in the middle of the drop (no double panic since it's in a different thread)
• The program continues with elements still remaining in the chan::Chan

This PR prevents such cases by enforcing a double panic on thread1.

[Darksonn]

Overall looks good, thanks!