Deal with timeouts for unreliable backends

[edsko]

The Deadlines guide states:

Deadline on the client

(..)

If a server has gone past the deadline when processing a request, the client will give up and fail the RPC with the DEADLINE_EXCEEDED status.

We don't currently do this (we rely on the server to respect grpc-timeout).

The timeout should apply even when wait-for-ready is enabled, see https://grpc.io/docs/guides/wait-for-ready/ .

[edsko]

See also discussion of timeout_on_sleeping_server in interop.md; we might need to update that once this is fixed.