Stop panicking in `MustUseOneshotSender`, send an error instead

[teor2345]

Motivation

In MustUseOneshotSender, Zebra panics when ClientRequests don't get answered by the connection code.

Sometimes that's a bug in Zebra, but sometimes it happens during shutdown anyway. So we don't want to panic when there is no response.

Designs

Stop panicking in MustUseOneshotSender:

• make dropped ClientRequests send an error response on the request sender, instead of panicking
• rename MustUseOneshotSender to ErrorOnDropSender

Related Work

This is part of:

• #3263

We could design the request code so that the compiler always makes us return an answer, but that needs a separate ticket.