net/rpc: wait for responses to be written before closing Codec

If there are no more requests being made, wait to shut down
the response-writing codec until the pending requests are all
answered.

Fixes #17239.

Change-Id: Ie62c63ada536171df4e70b73c95f98f778069972
Reviewed-on: https://go-review.googlesource.com/79515


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>