fix(proxy): proxy not answering before client timeout

This is related to #40230 (closed).

The initial MR that closed that issue, !391 (merged), was not semantically correct, because DataChannelTimeout starts after the client has already received the answer.

After !498 (comment 3156256) the code became not only semantically incorrect, but also functionally incorrect because now if this timeout is hit by the proxy, the client is guaranteed to be gone already. This commit fixes it, by lowering the timeout.

This addresses a suggestion in #40447.

This also closes #40381 (closed) and supersedes !415 (closed).